I shoot mostly 40 S&W and I have read a lot about bullet setback leading to overpressure and kaboom. So, here is my reloading question. What is acceptable variations in published cartridge overall length (COL)? For example, my load data calls for a COL of 1.125. If I have one measuring 1.120, 1.115 etc. At what point am I approaching trouble?

We've already established that I'm anal and I'd love to hit 1.125 every time, but I know that's not going to happen. I just want to make sure I establish safe habits.