Barrel length only determines the velocity. The powder choice is entirely determined by case capacity and its relation to bore diameter and bullet weight.

Ideally for max velocity and consistency, you want the slowest powder that will reach max allowable pressure for your given bullet. The slow powder will provide a full case which usually results in consistent performance. It will also sustain pressure the longest creating the largest "area under the curve". Area under the curve is what determines velocity so the ideal pressure curve reaches peak allowable pressure and sustains a high pressure down the length of the barrel. If the powder is too fast, it will reach peak pressure but it won't maintain that pressure and area under the curve suffers and velocity does as well. If the powder is too slow, it fills the case without reaching peak pressure and area under the curve suffers and the velocity along with it.

Notice that I didn't mention barrel length in any of this. For the several powders that suit a particular bullet weight and cartridge, barrel length determines the end result of velocity, but the same powders will perform regardless of barrel length. Notice the ammo makers don't sell loads tailored to different barrel lengths, there's a reason for this and that reason distills down to the fact that proper powder selection has nothing to do with barrel length.

Powder burn percentage isn't terribly indicative of anything either. Varget, for example, is usually at the top of the list for consistency and velocity out of the cartridges where it is suitable and it almost never burns completely in a practical length barrel. Doesn't mean anything. It is what it is.

Muzzle blast is determined by residual pressure when the bullet leaves the bore. High velocity loads will have high pressure when the bullet leaves the bore and will have muzzle blast. Unfortunately the two are part and parcel.