[Graham]

I would also say that on measurement of the Targa Florio cars, there is a good correlation between track clearance and wheel/ tyre diameter for most cars.
Those with 19mm or above wheel/ tyre diameter have 1.5mm track clearance, those with below 19mm do not have 1.5mm track clearance.

As I know builders will tend to work to the limits, I will make it clear that for this proxy (irrespective of previous ones) I will apply a severe penalty for exceeding the limits, so everyone should allow a little margin for error this time around.