The sprue, the part that is above the fill holes on the mold, usually weights about 1/4 of what ever bullet weight I am casting.
I somewhat over fill the sprue area to insure that the bullet base is completely filled out.
So when I cast I normally use about 30 lbs of lead to cast 24-25 lbs of bullets.
The sprue is then kept and used for the next batch of bullets.
Most of the time, since I cast so fast, I throw the sprue back in the pot as soon as it cools enough to open the mould. It can be done with no problem.