Old Loading Tables for BP did tend to state weight in Grains for the Powder Charge.
And, the concensus, has always been, to load impirically, as others have already said here - Load to where the Powder Charge shall be compressed well once all is said and done, however much BP that is.
This impirical volume of course, if weighed, will tend to be what the Books said anyway, BUT, it may be a little different also, if, say, one's powder has absorbed some atmospheric Humidity, thus, being heavier by weight for it, and, thus, having less volume for that weight, than the probably 'dry' as dry can be Powder used when tabulating the Charges listed in Books.
If wishing to use reduced Charges, then, Wads and or filler on top of the Powder, under the Bullet, and, making sure there is still good compression of the charge, appears to be the traditional and approved solution, and, definitely, never, ever, allowing an Air Gap to be present in the Cartridge.
I've been very pleased with 3F BP and a thin Grease Wafer for re-loading .38 Special.
Very good ( above smokeless 'saami' load spec for for same bullet ) FPS, nice healthy recoil, nice deep report, and, if breezy out as usual, I do not even notice any smoke.