Bart,
Not sure why you are needing a bore spacing measurement, are you building a double?
If so there are many here who would like to know more about your project I'm sure.
Regarding the bore spacing on say a double built from scratch, I use the "old four thou to the inch" as a rough starting point.
This equates to approx 100 thou pull in at the muzzels.
All this does is provide SOME convergence at the front and then the rifle is shot to see what happens.
Chances are the two barrels could shoot miles apart or miles crossed and each will usually be at different heights.
Make a decision on how much to alter the spacing after seeing where the gun shoots at 25 yards or less using 100 thou pull in.
Regulating from scratch means you have to start somewhere regarding convergence and I use 100 thou as my starting point.
First shots fired need to be at very close target range to ensure both barrels are on the paper.
Increase target distance as you improve the regulation.
Hope this helps.