People, you are over thinking the situation. The headspace gage that you should be using is what comes out of you rifle when you shoot it. Take those go-no go things and stick them up Hilarys rear end. A fired case tells you what the distance from the shoulder to face of your bolt is. If you have the Stoneypoint/Hor comparator system take out the fired primer and measure the distance from the datum point on the shoulder to the base of case. I use this measurement to set my sizeing die. Take you mod. case and then measure it the same way your did the fired case. Lets say that the mod. case is 0.005" less than your fired case. When you measure a bullet for seating depth with the mod. case you will need to add that 0.005" to the measurement to know were the rifleing is when you seat your bullets.