There's no such thing as too much coconut. :razz:
That having been said, for most recipes, I keep coconut at 10-15% maximum as I'd have to superfat at higher levels than I want to otherwise. There are exceptions.
Mathematically? Here's the rule I use. Take your percentage of coconut oil. Divide by five. If the answer is less than five, set it to five. That is your minimum superfat percentage.
I make a gardener's soap with 40% CO that I superfat at 1%--and test carefully for zap on each bar. But that's an exception to the rule.