fal_shooter...
I think what your talking about is the metod I use for setting up the top rails to accept the bolt carrier. I will be posting that operation tomorrow.
The critical locations are at the trunnions and center support. You want the top of the receiver to be almost the same (same would be great) width the length of the receiver. This is from measuring the outside of the receiver. What you don't want is to have the center support pull in or push out the sides of the receiver when you rivet/screw it together. If that happens you will have a gap when you put your top cover in place (it would be more noticable on the LH side of the receiver). Also your upper rail width would be uneven.
I've had the gauges for some time and they were really inexpensive for the job they do. Like a lot of tools used in building the AK, most are really not needed, they only make the job easier.
Here's a link to the telescoping gauges. $10.00 for a 6 piece set. Damn...that's cheap!!!
http://www.use-enco.com/CGI/INPDFF?PMPAGE=212
Hope this helps.