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Start with the gun manufacturer's recommendations, noting the maximum recommended charge, which should never be exceeded.  For shotgunning, use sizes of powder suitable for large-bored guns, essentially FFg or Fg in black powder, Pyrodex RS or Clean Shot, a relatively new black-powder alternative.  Finer powder, like FFFg, should not be used because of the higher pressure it creates and potential burn-through of over-wads.  (Powder for muzzleloading shotguns is gauged according to coarseness, which is indicated by the number of F's the finer the grain.)

There is a simple rule of thumb in black-powder shotgunning: powder and shot should be loaded in equal (or near equal) volume.  Simply use the exact same measure for both.  Roughly, a powder measure set at 75 grains will throw about one ounce of shot; 80 grains, about 1 1/8 ounces; 90 grains about 1 1/4 ounces.  Of course, different shot sizes will fill a measure differently so this is not - and doesn't need to be - an exact science.

For turkeys, try doses of 4, 5 and 6 shot at the range to see which patterns best.  I split the difference and shoot 5s.

Cabela's instruction manual calls for a black-powder charge of 92 grains for their 12-gauge shotguns; 110 for 10-guages.  (My 10-guage seems to prefer about 100 grains.)  A powder measure set at those levels will, accourding to the rule of thumb, throw the appropriate shot charge for the load.