^^^THIS^^^ I have a Magma caster & it will take 10# of ingots before it drops temps enough to stop casting. I always keep the pot at least 1/2 full.
I find preheating ingots on the top of the pot works fine for my LEE 4-20, just add them often, never let the pot get below 3/4 full. When I dump sprues pretty often. The time it takes to get back to temp is enough to allow the mold to cool a bit. Too much over thinking.