The 75% Rule, is the alleged regulation result that if you have a regulating bullet load, if you reduce the bullet weight tom75%, using the same powder charge, the lighter bullet will still regulate.

So:
.375 300 gr and 235 gr;
.400 .416, .423 etc, 400 gr and 300 gr;
.450 480 gr and 350 gr;
.470 500 gr and 350 gr.
etc.

I've never tried it.

Have you tried the 75% rule? Please report your results?