Out of the many ways to reduce regrind, this is the simplest.

Sprue lengths have traditionally been dictated by what is the standard sprue bush available in the market. Many moulders just let the toolmaker specify the standard sprue bush.
Often the sprue bush does not need to be the full length of the standard sprue bush. This is especially true if the cavities are shallow.

By shortening the sprue, we are actually reducing the regrind by a significant amount especially when the runners are short for example in a two cavity or a four cavity, 2 plate mould. One can appreciate as any reducing in length of the sprue is taken off at the larger end of the sprue.

For sprue bush with a 3.0mm orifice, and 3 degree total taper, 75mm length, a 20mm reduction in length of the sprue bush is a reduction of 42.5 % of the volume of the sprue hence less regrind as indicated by the sketch below.

As a rule the sprue should be made as short as possible to reduce regrind without jeopardizing the other considerations for a good moulding.