I think it's simpler to give the KHH a set multiplier. Piccolo pointed out that Gohan's ki being 50% was the reason for his KHH not matching up to Cell's.
Chapter: 415 (DBZ 221), P10.2
Context: after Gohan fires his Kamehameha
Piccolo: “It no use…! Gohan’s ki is weaker after all…”