If you look at the ballastics sheet that should be included with your bullets, it should tell you how many grains of powder for each grain of bullet. The hornady bullets for example;
240 xtp;
using pyrodex powder at 90 grains, leaving the muzzle it will be .810 low
at 50 yards it will be 2.25 inches high
and at 100 yards it will be dead on
If you used this you would know where your gun was going to shoot at these yards, and it also helps if you are putting on some type of scope to get it where it needs to be.Check your box and see if they reccommmend anything.
I don't know for fact, but alot of people have said anything after 120 grains, then you are just burning it out the end of your barrel, and just waisting powder.
I have two Knight 50's and one is on two 50grain pellets, and the second is on the new triple seven sulfer free at 90 grains.I am only using the pellets for my son because thet are easy to dump down the barrel.After this year both guns will be on the triple seven.What a difference there is in cleanup.Triple seven takes only a fraction of the time.Smells better too.