General rule of thumb for SEP.
Each 50 RPM reduction gives about a 100 fpm descent.
For CS prop, 1" manifold pressure reduction achieves the same effect (approx)
General rule of thumb
Dividing ground speed by half and adding a zero to the result gives the descent rate in fpm to maintain a 3 degree approach - please note that I am not an advocate of 3 degree approaches in SEP.