A frosty bullet is a happy bullet. Keep temps up where you cast nice bullets and not higher than 700ºF. Cooler is better for the alloy as far as oxydation and separation, and some odorless, colorless, poisonous gasses will be emitted if melt temp is too high, but frosty is OK. Some moulds will cast well at 600ºF, some at 700ºF ( 'bout all of mine do best with WW at ~675ºF ). If your pot is big enough, an $8 griddle thermometer from the kitchen store will float nicely on the alloy(they get kinda smokey on the inside of the glass, but seem fairly consistent and still readable). Or you could spring for a nice $30 probe one.