You'll have to do some experimenting... it depends on how hot your mold is (your overall production time and speed factor into this), what size cavities you are filling, and the fact that the rheostats controlling temperature have some variance from pot to pot. For comparable jigs, I usually start casting with mine at 7, but then turn it down to 4 after everything is up to temp. Again, you'll have to experiment, but if you get too cold, you'll start to see ripples in your casting.
For really big cavities, you'll want to start higher/hotter and stay higher/hotter... you can also (or may need to) pour using a ladle or directly from a tippable pot to prevent rippling.
