The easiest thing to say is to follow the loading manual.
After you get that part down, you can then branch out a bit.
A lot of what people call "unburnt" powder isn't what it seems.
You didn't specify if you're referring to rifle, pistol, or revolver, so a general blanket will have to cover. It's best to use a powder that will fill your case in such a manner that a double charge will overflow. Maybe a bit messy, but better than a blown firearm.
I make up a dummy cartridge with a bullet that was previously colored with a magic marker, to see how long to seat my bullets in my rifles. It can go from ridiculously long in my NEF, to a specified length in my re-chambered 30-06 a.i.
That works well for me, so that's o.k.
Remember, this isn't rocket science, you're just safely launching a missile. ( O.K., maybe it is...)
But the best thing for most people is to follow what's printed in the most current loading manual that you can get. The powder manufacturers have theirs, and the bullet manufacturers have theirs.