There are no rules of thumb.

If you are designing your own plane, be prepared to adjust your dihedral during flight tests. More dihedral is needed if adverse roll is present in knife edge, that is it rolls in a direction opposite rudder. Less dihedral is needed if the aircraft rolls in the direction of the rudder. Due to fuselage geometry, etc., determining exact values of dihedral before flight is pretty much black magic.

If you are building an existing design, go with the suggested values, but be prepared to tweek dihedral mechanically or add electronic rudder-to-aileron mix.