Sweet Spot Tips For The Lee 4 - 20 Melter And New Lee 457 500 R Mold

Of course, a PID temperature controller would be ideal, but in lieu of a PID, what did casters of the past do to get good bullets? It's called getting in the sweet spot when good bullets are dropping out of our molds. When we get into the sweet spot with our new Lee 457 500 R mold, how do we stay there for the duration of a 3 1/2 hours session?