If the radius is 15 cm or less (which would equal a diameter of 30 cm) how does this relate to rule 3e - the maximum diameter is 25 cm.

The 15cm referred to in rule 3.f is an exception for single-bladed rotors only, as they have less lifting area and their swept diameter is extremely difficult to measure. Any rotor assembly with more than one blade must have a diameter less than 25cm. If a rotor only has a single blade, it must have a radius less than 15cm.