I use the RCBS case mic designed to check setback and bullet seating depth. The bullet seating depth is a bit cheesy, close to useless, but where the RCBS shines above the Wilson is all in the numbers. RCBS will tell you down to the .001" where the headspace is in relation to SAMMI. A cursory inspection of unfired factory ammo on hand shows HXP about +.003", WCC 56 tracer -.002" LC67 .000", LC69 +.001", Korean PS 75 -.002", and Rem, Fed, Win all .000". Yes, the RCBS is slower to use than the Wilson, but if you have a quality press with no slop, once the die is adjusted you are done with the case checker.

If I adjust my shell holder against the sizing die, I get a -.005".

The best thing to do is get the RCBS tool and measure a new factory round then shoot it out of your M1. If you are OCD, remove the gas plug so you know the case didn't deform in extraction. Now measure the fired case and compare to what it was before you fired it and where it is relative to 0 on the gauge. Now you know how much the case stretched in firing. Shoot a couple more and check them then adjust your press to the shell holder hitting the die and measure that case.

A book could be written on what to do with that data, but suffice to say if my original case is .000" and my fired case is .003", I am not going to set the full length sizer to touch the shell holder if that gives me -.005", I'll adjust mine to .000" so I don't overwork the brass.

...almost forgot, if the inside of the neck has not been lubricated, the expander ball will stretch the case on the upstroke.

I lost a lot of hair when I first got my case mic learning the above. I hope you have a shorter learning curve!