I echo Amir's sentiments. I make it a "rule" in a .308 sized case that if the max load for a given powder is below 41grn, it's too fast. If it's above "about" 48, it's too slow. To me those too fast and too slow phrases translate to "not optimum for the bullet weight".
Light weight bullets in a .243 respond well to quicker powders. The slower powders start coming into their own at a bullet weight of 85grn +.