After going through a pound of 2400 (my very first pound of powder, ever) in my .357 Dan Wesson (my very first gun, ever) I calculated the cost per shot and then searched out the most economical powder. Bullseye powder. It was the most energy-dense (by weight) powder I could find. I did a few pounds of that and then switched to Unique because a powder that energy-dense is just scarey! No big worry, I always have loaded mid-range to light, so was pretty safe. But a little tiny bit extra charge and you get higher pressures and more velocity and a tiny bit less gives lower velocities. Does not do much for accuracy.
When my friend bought his 500 Smith I suggested the least energy-dense powder (by volume) I could find (Trail Boss as it happens; it may not be the "fluffiest" powder out there, but getting the data to choose from was difficult and I had to pick from what was on the shelves).
For practice and familiarization, Trail Boss has been GREAT! 325 grain bullets at 800 fps feel like shooting 22 rimfire out of a gun that heavy. We do shoot heavier loads with other powder, each one appropriate for the bullet weight and velocity we are seeking. We do have one 700 grain bullet we will probably never fire, but we have put a number of 375, 400 and 500 grainers downrange at full power, each over an appropriate powder and charge. The method: Select a bullet. Determine a desired velocity. Go to the manual and find what powders give that velocity at near 75% to 80% of maximum charge (if there is such a powder) and use that one, or as near to it as we can find. If he does not have that powder in his shelf, he buys it. It's not like it's going to spoil in a month. Then, next time at the range, we chronograph it and see how it's performing.
There is another consideration than getting the case full enough to prevent a double charge. Powder position matters.
When you don't fill a case all the way up, the position of the powder in the case makes an appreciable difference. We found that we can vary the velocity of the bullets out of his 500 from 750 fps to 825 fps simply by varying how we hold the gun. Point it up and bring it to level before firing puts the powder at the rear of the cartridge and gives 825 fps. Point it down and bring it carefully to level before firing puts the powder at the front of the cartridge gives as low as 750 fps from the same batch of reloads. I forget if we were using magnum primers or standard, though.