jmfa1957:

According to my Redding Full Length sizing instructions after you screw the die down touching the shellholder with the ram at its highest point, one is then to back off the die half a turn and tighten. Since the die will not touch the shellholder one cannot square the die that is why you place a washer/spacer between the die and shellholder to take up the difference of the counter half turn. While keeping the ram raised and every thing in contact, you can now tighten the set screw on the lockring and your die is squared, and you will be full length sizing because you followed the Full Length instructions.

With RCBS dies using a washer/spacer is not necessary because one screws the die down a quarter turn after the shell holder is reached, so applying pressure with the ram and tightning the set screw will square the die. If you are using a RCBS Rockchucker press, one can cam it over making it easer to tighten the die's lock ring set screw.

With Forster Dies one screws down the die till it hits the shell holder firmly, so the die is squared after the lock ring is tightened.

I square all dies my dies (full length , neck sizer, seating, and case mouth expanding).

I don't own any other dies, so I haven't squared them.

Good luck.