Like most web sites there is some stupidity on "Cast Boolits". Cooling the sprue plate is right up there.
Generally speaking a bullet like a REAL bullet should be no harder than 1:20 tin lead. 1:40 to 1:16 will cast perfect bullets with a low reject rate even if weighed to +-.5 gr if the casting is done right. The tin lowers the temperature at which the metal will flow.
Generally pure lead and tin is the best alloy fro BP, either in MLs or BPCRs. Though I have had moulds that were poorly designed and needed near #2 alloy to shoot well.
The key is in the casting process, use hot metal, cast as a speed that keeps the mould hot enough but not too hot and segregate bullets by casting session. 2 different sessions with the same alloy and same mould will often be off enough to increase the number of culls when weighed.
Note that I am referring to match grade bullets but I use the the process for all cast bullets even RBs. Its not much more work and the bullets are more uniform. I relax the weight requirements when casting pistol or light weight rifle (hunting) bullets from a multi-cavity mould.
Bullets that are more than 1 grain light from the same cavity are useless for precision work.

Dan