Alberta- you need to pick up a Lyman shotshell loading book. There are a bunch of loads you can use for base data for developing your own data.
; They list loads form .690" balls to 575gr. Brenneke slugs- in most years of manuals.
; You can extropolate from that data. Always start at the starting load or a few grains (weight) of powder lower.
; A chronograph is useful when developing data.
; Once you get those .715" balls (545gr. in pure lead) to the 1,450fps to 1,500fps range, you'll have close to the equivalence of the 7 dram BP 12 bore load.
; The suggestion of Unique is a good one, but quite soft.
; I'd start around 24gr. and go from there.
: Most ball loads with 7625 and 4756 will end up in the 30gr. to slighlty higher range - BUT - should start around 24.0gr. - whatever is safe with a shot charge of the same weight. The ball will develope slightly lower pressure.
: Remember to use a gas check off a trap of other wad to centre the ball - cup up, of course and one, cup-down on the powder - fibre wads in the middle. Lyman also has some suggestions.