I used to try a number of powders and always ended up with the powder that came closest to filling the case. So that's where I start unless I want to play around with faster powders.
Another advantage(?) is that with the slower powder you get higher muzzle pressure which makes a bigger boom. Used my 6mm Rem. when hunting with my cousin and his kids one year, hand loads of course. Don's kids were using 243 Win with factory loads. The kids were mightily impressed with the 6mm because with that big boom it must be much more powerful than the piddly little .243.
