Except no structural point of the car can come in contact with the road. Take NZ roads into account with your theory there. All the stupid bumps and camber etc, and 40mm travel = 80mm ride height to avoid chassis slamming the tarmac.
TBh I think getting someone to cert a car at 60mm would be pushing your luck. Ultimately it depends on the certifier, and some will be more open than others, but I've heard of a few cars in the waikato being rejected as being "too low" at more than that. Suspension geometry and bump steer issues could have also played a part in that, mind you