Thanks for the tips, surely it will help those using the tool. The way I decided to go is both free and doesnt ruin a case so its too late for me. I find it strange they used such an oddball thread.
There are two ways that you can eliminate the headspace variation.
The first is to use the Hornady headspace gauge to measure the difference between the base to datum line on the modified case and your fire formed case.
The second is to take a fire formed case which has a loose primer pocket and drill and tap it to be used as a modified case.
Use a "K" ( 0.2811") drill bit and a 5/16-36 tap.