It's more of an art than a science for sure. It depends on the log species to start with, if it's green or dry as to where I think I want the speed to be. Harder and dryer, or twisted grain - going to be slower. Another big consideration is how knotty do I think it is, more knots then again it's going to be slower. Once I have an idea of where to be I listen to the mill, I like to keep it right on the edge of the governor so she is pulling hard, but not bogging down, I also keep an eye on the portion of the blade I can see by the movable guide, if it begins to dive I need to back down. Another good indicator is your band tension, if she is getting hot (dull, too fast, not enough lube) then the tension will begin to decrease. My right hand basically never leaves the speed control so I can instantly back it off if I need to.