Beekeeper,
You certainly will not go wrong with a cork filler (or something that is firm) or a full case of BP. The US Calvary 45-70 load, as I recall, used 55 grains of BP with a cork wad over it to fill the airspace. I do agree with BigTed about letting somebody else do the airspace experiments. I am a firm believer in using a full case in most cases. Having a gun get ruined or blown up (and maybe wearing a "steel mustache") just does not sound like a good time. Everyone agrees that a full case of BP is safe. If recoil is an issue, there are ways to deal with it other than reducing the load.
Longwood is right about charred cornstarch being added to BP when it is being made. I do this when I make BP, as it helps the kernels bind together and not crumble.