Re: Headspace guage
That's all there is too it. get the propper collar to fit the shoulder of the brass you are measuring. It touches the datum line on the shoulder. With your calipers, you measure from the head of the brass, to that datum line.
I like to measure unfired brass, 1x fired brass and 2x fired brass (all unsized) to see wher the shoulder ends up. It depends on how tight your chamber is. As stated, do a partial full length resize, bumping your shoulder back about .002". I like to do this starting with 2x fired brass. I feel that it gives a better blueprint of your chamber. The 1x fired brass gets neck sized only.