One way to approach this is in relation to a chamber of known dimensions. It is common for benchrest gunsmiths to cut the front of a chamber into a short piece of barrel, that is included with new barrel jobs. If you have access to one of these in your caliber (bullet diameter) , you can find the point at which your preferred bullet(s) touches and then build another dummy round that has the bullet(s) seated where you would like. By comparing the lengths of these dummy rounds, you can come up with how much you need to shorten or lengthen freebore so that bullets are in your preferred relation to the rifling when seated to the length that you want. For instance, If someone wanted to design a 6mm reamer, we could use my PPC barrel stub, a couple of sized cases (PPC) and a couple of the bullets that he wanted to use, to make up a couple of dummy rounds, one with the bullet positioned as he wants, and the other with the same bullet seated to touch the rifling. If for instance he is building a rifle for a 6BR with a fast twist barrel, and wants to shoot a bullet that is in the 105-107 gr. range, we can use one of those bullets for both dummy rounds, even though the case dimensions are different, the difference in loaded lengths will tell us what we need to know, since I know that my reamer has a .060 free bore.