Re: getting different OAL's each round?
Even the very best custom bullets vary somewhat in length. I don't like the comparator for the very reason you cite. I prefer the Davidson base and nose pieces available from Sinclair. The nose piece stops on the ogive, giving very accurate OAL readings.
With dies equipped with a micrometer top, I like to seat them about .005" or so high. Measure each and set them up in a loading block, mark each line in one thousandths increments. Then lower the mic top one thou at a time to hit the exact OAL right on the mark. If you do not have a seater with mic top, you cannot do this.