What Clark says is very accurate...if the shellholder and die are bumping different rates of cam over can make the redding comp shellholders a little fuzzy to work with.
I use a hornady head space gauge and a rcbs head space mic to guide me. I've found different brands of brass might size different, harder brass after repeated use or annealed brass will also size a bit different.
I've used in the past the comp shellholders, and skips shim kit from Sinclair to make adjustments. What's working best for me now is a shaved shellholder that doesn't contact the die. I put a matching index mark on the die lockring and theads that represents about .002" head space for most brass. If I have to make an adjustment one way or another moving the marks apart about 1/4" corresponds to about a..004" change in head space. So basically I'm back to moving the lockring to make adjustments...but my marks make it fast and pretty accurate. Funny how you can come full circle sometimes.
Also re head space....I used to cycle the case through the sizing die pretty fast....I now hold the case up in the die at least 2 seconds... i feel this gives me a more uniform sizing...but not sure I can prove it.
