I think you need to judge it based on weight, charge by charge.
How many grains of BP you'll need is going to vary, since you don't actually weigh it; you "fill the case." (There are detailed instructions elsewhere.) But let's consider the .45 Colt, originally loaded with a nominal 40 grains of black gunpowder. In modern cases you won't get that much, but as an arm-waving approximation it isn't too far off. Substitutes go by equivalent volume, not weight, so if you're talking about a BP substitute it won't be exactly the same weight either. But it will be somewhere in that ballpark.
According to my reloading manual, a fairly normal .45 Colt smokeless load would be about 8 grains of Unique. This is an example only, as the charge weight will be different for each smokeless powder you might choose.
So at 7000 grains per pound, BP will give you something more than 175 .45 Colt loads. Unique will give you around 875. Those are general approximations only. The actual data is going to vary depending on the volume of the case you're filling, what caliber you're loading, and which smokeless powder you choose for your comparison.