I'm not absolutly sure that I understand what you are asking but I'm guessing you are talking Ranger style gunbelt and main body over-lap. Is it possible that you are working from the Ghormley patterns, as thats the most common reference I've seen for chape? If you have the chape/buckle and the billet figured out and a waist or hip measurement to work with just position them on the body of the belt in their correct location. Remember that the actual measurement will be from the buckle to the center hole for the fitting part of the project. If you are asking what I think you are, now mark the overall length of the belts main body to be even with the outer bellet tip on one end and the outer buckle edge on the other. This is not a hard set rule and many do vary for artistic appeal but this is the most common. If I'm wrong in what you want ask again with a bit more detail.
The Chicago screws are ok for buckle attachment and I actually like them better than the common snaps but for a more awthentik approach just use the stitchin alone for attaching the chape and billet to the belt body and copper rivets for attaching the buckle to the chape. Naturally matching copper rivets could also be used to reinforce the stitching if you so desire. Once again, I like and use Chicago screws, with blue LocTight on the threads, for most of my work though.