While I wouldn't say this is compatible right off with the 12 bore, it is interesting and shows the powder's incredible versatility. They say any rifle cartridge, I'd ask IMR it might apply to your 12 bore too and would make the job far easier. It produces exactly the velocity ranges you'd be after too. Below is IMR's formula for calculating max and starting load in any rifle or pistol cartridge, hopefully IMR can clear up if it'll work for your purposes.

http://www.imrpowder.com/PDF/Trail-Boss-data.pdf