Well, there's kind of two different questions in here. First is... what is the diameter of cartridge XXXXX. And the way most of us get that answer is... we just kinda know it.
Like we know that .30/30, .30-06 and .300 Win Mag all use the same diameter .308" bullet. We also know that some are a little odd... like the .303 British is [i]not[/u] the same as our .30 cals and that the 8x57, while referred to as the 8mm Mauser or the 7.92x57 is going to require a .323" bullet.
load manuals will tell you this, but not all of them I guess.
The other question is what bullet diameter to I need in cast lead for my handgun? And the basic answer is one-thousandth over the typical size (as you said) but more advanced loaders and casters will tell you to slug your bore and find out EXACTLY what size bullet you need for the best performance.
Slugging your bore involves driving a soft lead ball down the bore and then measuring the diameter of that crushed ball to see exactly what your particular handgun would like best.