And Here are my edits of Hoot's description of using the Lee FCD Die for Side-Crimping the BM450. It is much clearer if you have the FCD in your hand, while you read these.
This is how I set up a run:
1. Have some brass not worthy for reloading for whatever reason, handy to serve as test indicators and a Sharpie.
2. Run the set nut on the FCD up as far as possible. You can get them up a little further if you put them on upside down.
3. With the shell holder in the ram, run it up all the way and screw the die in until it contacts the shell holder.
4. Retract the ram a little and screw the FCD down one revolution.
5. Looking down from the top, at the collet fingers inside the FCD, run the ram back up all the way and you will observe the collet fingers constrict a little toward the center at the top of the stroke. Repeat screwing the FCD down further until the collet fingers do not constrict any further and the gaps between them are closed.
It doesn't hurt to cut some thin strips of paper towel and pull them between the gaps in the fingers with the collet relaxed from time to time, to get any junk out from between them. Edited by Hoot If I made your FCD, do not spray degreaser into the FCD to clean it, or immerse it in a cleaning tank. I put a small amount of MoS grease inside where the wedges of the collet fingers engage the wedges of the body. It keep the collet from seizing. You don't want to wash it off. Shooting a little compressed air down inside to blow out any debris is fine. The un-modded die ships with no lubrication.
6. Having now set up the FCD, mark up the last 1/4 inch of one of the test cases with the sharpie. Put it into the shell holder and raise the ram just enough to see and feel the collet fingers begin to engage the case. Don't use any more pressure than needed to imprint the sharpie ink and you'll be able to reuse the case. That may take some practice. Run the ram back down and inspect the case to see where the crimp line is located. If it's where you want it to be, lock down the set nut and you're ready to process your preloaded rounds.
7. Taper crimp all the rounds after you apply the FCD and don't taper crimp them as heavily as when you are using just a taper crimp alone. It mainly serves to de-flare the case mouth.
8. If you need to move the crimp to be closer to the mouth, measure the amount with a calliper and use the shim washer, or combination of them to get that change in distance. It may take a try or two to get the combination figured out, so always have the sharpie and test cases close at hand. When you add a shim, you need to repeat the set up procedure. The end of the bottom of the collet engages the shell holder when you run the ram up and presses the collet up into the FCD body where the wedges of the fingers and body force them to deflect inward. That's how it works. If you add a shim, it's like screwing the die body down past where the fingers are fully engaged, so you need to back out the die to offset the change in height. The shell case has no bearing upon how the FCD mechanics work. Again, when you get the crimp located where you want it to be, screw down the lock nut. When using shims to move the location of the crimp, you have to slide them down over each round before you crimp it and remove them when you pull the finished round out. It adds a step to the reloading process, but that's how it's done. The alternative would be to have several modified FCDs cut to preset lengths and just use the appropriate one for the crimp location you need. That gets expensive, but it eliminates the need for shimming.
As you said, playing with it will give you a better understanding of how these factors interact. If I was not clear on something, please ask more questions. Having covered the "Lion's share" of the process, the answers will not be so long.
Life Member, Texas State Rifle Association; Patron LIfe Member, NRA