I sharpen the back edge of the sprue plate.
I set the sprue plate loose and cut the sprue high.
Then I slide the sharp edge of the sprue plate back over the bullet shearing off the base.
You do it as soon as the bullet starts to harden so it is still soft and will not bend the base inwards from the shearing action.

The process results in the bases always being the same height without a high sprue, shears off any base fins, and shears it off without any divots occurring.
This results in square edges and more consistent bullet weights.

This method produces bullets with more consistent base edges which result in more consistent bullet tension in fixed cartridges.
Of course a sizing die also helps provide a more consistent base diameter as well.

I also rub the top edges of the mold with a stone to create an air vent (between mold halves) which allows the bullet to fill out better on the base since the air is not trapped under the sprue plate.
Picture of mold air vent after bullet base sheared.

I use oven mitts inside of leather welding mitts so I can handle the hot mold longer during the shearing process.