I'll 2nd the suggestion to get the Lyman Cast Bullet book. Another good one is "From Ingot to Target" that can be downloaded for free.
Get a casting thermometer. I cast a long time without one, so it can be done. Knowing the temperature removes some of the guessing.
Clean your mold before using it. I use Brakecleaner and an old tooth brush. Also lube the line-up pins and sprue plate.
Pre-heat your mold. A $12 Walmart hot plate works well for this. Also, pre-heating your ingots before putting them in the pot reduces your wait time.
With practice you should be able to reduce the weight difference by quite a bit. The difference in weight between my pistol bullets is real close to what Sierra bullets weight difference is. I'm talking 10ths of a grain, not grains! So, it can be done.
