I've found different brass or different number of firings leads to (slightly) different fired measurements with my Hornady kit. I generally set up to bump 2 thou back from the measurement of the brass I am sizing.
I'm not sure if I am explaining that right, so here's an example:
Brass x reads 1.840. I'll set my body die to bump to 1.838.
If after the next firing the brass reads 1.841, I'll bump to 1.839.
I figure there has to be some inherent measurement error, the Hornady kit isn't "true" headspace, etc. As long as I am going 2 thou back from the measurement that just came out of the chamber, I assume I'll be OK. Haven't had an issue yet (FWIW).