I've been handloading for 35 years and I still haven't found an answear to this question. I have a lot of different cans of powder, with varying amounts left in them. Having bought 'THE' powder for a particular caliber, only to have it give poor resualts in my gun, it then sits in the cupboard, sometimes for years, untill I find a use for it. I don't like to do this but it happens fairly often. When I start to load for a new caliber, I try to do as much research as I can. I look through my loading manuals, old Handloader Magazines, Guns+Ammo, and Shooting Times. Ken Waters 'Pet Loads' are good, but are sometimes a little dated and may not list some of the latest powers. But many times the older powders will work as good today as the did 20 yrs ago. If I see a powder that is said to work good, in a number of sources, it is a good place to start. If I see a powder that may not be the best powder listed, but I already have it and don't have to buy it, I will try it. It may not be the optimum powder, but it works good enough to hunt or plink with, and I can use up a can of powder that is sitting, doing nothing. Sometimes the performance is great. This forum is a great place for information. Post the caliber and what bullet you want to try, and someone will usually have a favorite powder. Some calibers will have a powder that will work in almost all guns in that caliber: IMR or H 4350 in the 30/06 and IMR 4198 in the 222 R. are two good examples. Some calibers, every rifle seems to like a different powder. I found this to be true of the 22-250. I've loaded for about 8 or 9 different 22-250 rifles and almost every rifle liked a different powder. A powder that gave great resaults in one rifle would give poor, not even acceptable, accuracy in the next rifle. I hope this helps. That's best I can tell you, it's a trial a error process.