First . .
Please... just get a lead thermometer.
Don't try to do anything by eyeball, by guess-and-by-golly, or by number on the dial.
You'll drive yourself nuts -- no matter what others might say.
(700-800 degrees The lower number for Lyman#2/hard alloys, the higher for nearer
pure lead/soft alloys)
Second, get/use a covered hotplate for prewarming the mould while the lead is melting
(On that one, you can set the dial sorta in the middle)
See:
https://s15.postimg.cc/n7t2rmxtn/Hotplate.jpg
Third, no matter even pre-warmed mould, the first half-dozen casts will require a wood or
rawhide hammer to be used to open the sprue plate. (Just make sure to whack things
absolutely flat/in line with the mould's top surface so not to bend anything over time.)
Fourth, even when up to temperature, aluminum loses heat quickly, and so requires you
to cast fairly fast to keep it up to that temp.
All this sounds complex -- but a couple of sessions and its like riding a bicycle.