Shrink is the offset height x tangent (bend angle/2). This is also used to determine the spacing added on for smooth looking parallel offsets. The Shrink multiplier is literally the length of the hypotenuse minus the length of one leg of the triangle "distance to the obstruction", for a right triangle with a leg of length 1. If you want to do the math crudely; draw up 5 right triangles of 10-22.5-30-45-60 degrees as the known angle; and make the height of obstruction 1 inch. Then solve each triangle, then subtract the unknown leg not length 1 from the hypotenuse. Then compare your answer to tangent (bend angle/2). The entire multiplier/cosecant method when bending offsets etc, that's a go around to the Pythagorean theorem a2 + b2 = c2; I can elaborate later; we instead use the offset height x multiplier (cosecant) method; which works when shrink is incorporated.
I also have a table in my hardhat so I don't have to revert to using calculators.
As for physically hand bending. Take these tips as you may. They might work, they might not work for you. So you bend a 90 have a stub (a shorter section of known length), and a leg (a longer section of irrelevant length). The longer length of the leg means an easier chance to bend. One more short fact. There are essentially two types of bends. 90's and triangle bends (which can be more accurately bend on center). This can be expanded later.
You can bend overhand with a hand bender; I've come to prefer all my offsets and saddles in the air then on the ground. There's nothing inherently wrong with bending overhand. Just make your motions in one fell swoop. For example, I've seen people struggle bending 1 inch 90's; I do one jerk to get it around 30-45 degrees, then do the 3 reach rock. One foot on the ground, dominant foot on the leg of the pipe, one hand on the handle of the bender, one hand on the stub length, then I make one smooth rocking motion to complete the 90. Just one fluid motion, no matter how you do it. Caveat: 1-1/4" EMT offsets has to be bent on the ground (I've always bend the second half of it on a scissor lift). I haven't fucked with 1" RMC with a hand bender.
Offsets greatest obstacle are dog legs. Lay the doglegged offset on the surface so that misaligned bend is off the ground, so it creates a "hump". Like in the cross-sectional view (caret symbol) instead of ("v", valley). Stepping on the travel section/hypotenuse/bend 2 bend, pick up the leg which is not aligned. You can use a screwdriver and pick it up, or use grab it with your hands and lift. That should correct the offset. Video: https://www.youtube.com/watch?v=7MQRqFEORyw . There are other methods of correcting offsets, such as placing them back in the bender, but I found them ineffective. People often put offsets at the end of the pipe because they can hide any dogleg easier and shrink calculation can be avoided; cut EMT or cut and thread RMC the other end with a maizeall .
If you overbend one section in the offset, correct the second bend. Correcting the first bend is more complicated and will often affect the second bend.
A 3pt saddle right next to a 90 is a challenge. Bend that saddle first, then lay the saddle flat, and using the star, you can bring the saddle up to create the 90.
If you can't bend 90's right on your first try, it's easier to overbend 90's then correct them. I use two torpedo levels, one to determine if I've reached the 90, the other perpendicular to make sure I'm aligned straight up and not leaning any one side. I use both at the same time for a simultaneous reading to make sure I get a dead on 90. It works for me, and I can bend dead on 90's reliably, quickly, without being a 4 to 20 year veteran. It also helps to correct bends with the conduit bender shoe still in, instead of rocking it naked. The latter has a potential for kinks.
When hand bending kick 90s, I have 3 methods to bend to the kick height, instead of shooting a tape measure down every time and guessing. If I have a carpenters speed square at hand, I'll use that for a kick of less than 6 inches. The carpenters speed square stands up. For a time I've used the tape measure and stood that up with the tape shot out, to know what length to bend to (because I know the inside length of the tape measure -- usually 3-1/8th"). That works, but only in a pinch. For my go to method, imo, is to cut a scrap piece of the length of the kick, then stand that up. Once the bottom of the pipe lands on top of that scrap piece, then the kick has been bent at the proper height.
Kick 90's will measure the same kick height no matter how you flip the pipe. If I'm bending a kick 90 with a longer leg to be kicked up, it's easier for me to flip the 90 over to measure the shorter stub and make sure that kick height is level/accurate. Correcting kick 90's are me just stepping on the bend to level it out.
Installing kick 90's work best for me when both legs of the run have been already been installed and are aligned. Then I would just fit in the kick 90. This is easier for me than installing it from point A to point B, because it's hard to align a kick 90, then hope both legs will end up looking parallel and neat. So I do point A straight, and jump to point B and do that straight. Then the intersection is the kick 90; where I know the kick height, and have presumably already bent that kick 90 on the ground. Then I bring it up, I can cut to length on both sections, and then install into one open coupling and hopefully one sort of support.
Working with racks, my tip I learned through reddit is to intuitively kick down from the rack then make your 90. If you make your 90 first, then kick down, you'll cockblock everything else on the rack. Yes, this is where moving from 1 rack of one elevation to 1 rack of a different elevation, I finish point A straight, point B straight, then cut in my kick 90's.
When bending 90's with another bend in the pipe; back to back 90's etc, I recommend the star. When using the star, your bender's hook (you) should face a flat piece of unbent pipe.
If you bend 45's on center (the notch), and can't do it to too short a leg length, you can flip the bender around and bend on the stub. The 45 on center should be symmetrical no matter which way the bender is facing.
If you're bending a 90 on the star, and can't complete a bend because the leg is to short, you start the bend on the star, then turn bender around, and finish bending the other side. It worked for me on 1 inch and 3/4 inch EMT.
If you get the math right, it's easier to bend rigid (RMC) than EMT because EMT kinks so easily. If you mess up a bend on rigid, you can beat the crap out of it, and it can still be presentable. That said, it's way way harder to install rigid (RMC), especially surface mounted 90's without threadless couplings/connectors. It's doable but requires taking the previous 90 off the wall and spinning on your piece first. For that matter, I only have limited experience installing rigid.
You don't have to be 100% perfect with your bends. The structure which holds your pipe will bend the pipe to where it needs to be. That means a way more challenging install with improperly bent pipe. But if you are 100% perfect with your bends, it will make a much, much easier install. The pipe will essentially support itself. And it's easier to bend the pipe correctly than installing some fucked up shit in the air, and hoping it corrects itself.
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