Surprised you haven't been inundated with answers yet. I was taught to use the following on English 12 bore Game guns.
Pre 1925 Proofs
Chamber Immediately prior to Forcing Cone. .105"
9" from Breech .045"
4-8" back from muzzle .025"
Post 1925 proofs:-
.100"
.040"
.025-.020"min
Smaller Gauges May require a little more.
Hope this helps get the discussion started.

Hugh Lomas,
H.G.Lomas Gunmakers Inc.
920 876 3745