In our case (F16 class) the formula used extensively was the Texel rating formula where the result wasn't some 12 meters but rather a performance number (The F18 rating as a matter of fact).

Later in the class history we simplified the rules to a more box-rule type of of class. Example; there used to be far greater freedom in sail area and luff length then now; then we used a formula to calculate a rated sail area and only limited that. Now we have simple max limits in area and luff length.

Other then that I agree that we are all (the F-classes) more a box rule setup then a formula setup. We choose the F-name in order to follow in the footsteps of the F18 class. Not much thought went into that particular aspect of the class.