In India-region Toradars with their special, elongated ‘firing chambers’ that narrow down (bore restriction) right below where the ball sits, some use fillers of corn meal or even cheap oatmeal to fill up that lower space. This is needed for 2 reasons, as (1) the powdah back then was mealed and wasn’t as dense as 18th century or modern mixes and it needed to be tightly packed (hence the bore restriction/chamber ‘ring’) but (2) also “to be kind” to an old original where one wants to shoot it, but using a lesser charge.
If you have a straight bore, I offer 2 other suggestions, perhaps even used in tandem. You could try a good 1/2” thick fiber wad (lubed) above the powdah to take up some space and/or use 1Fg powdah. Now my flint smoothies tend to have long barrels (up to 60”) and bigger bores (65 to 75-cal) and on a suggestion, I recently tried Goex 1Fg and the results, for me, were phenomenal!
I took the big 75-cal 60” barreled club butt to a recent woods walk and not only won the smoothbore contest, but was only a couple points way from the highest rifle bore score, using French type paper cartridges and 90-grn loads to boot! Not only was the accuracy outstanding (placed 2nd in smallest group contest) but I shot it 30-times or more without swabbing the barrel once.