You mean you actually what to know what powder you are loading into a given cartridge?? What's the fun in that? Actually, I totally agree with your point. If I were ever at the point of pouring powder "X" into a case, I would go the other way round (put the powder in a large volume shell) and experiment very carefully.

It is ever so much easier and effective to keep a store of correct powder and primers for the firearms you possess.