I don't understand the physics. The idea of landing within 60% for turbojets or 70% for turboprops I believe is to allow for pilot error with respect to speed control and landing technique. It is intended to prevent a slightly high energy approch resulting in an overrun or a prologed float having the same result. Energy varies with respect to the square of the speed but the factoring is a simple factor. So the legislation means that the faster your aircraft ref speed is the smaller margin of error the legislation allows.
Which is wierd. It means the rules allow you to be an untidy pilot if you fly an aircraft with a lower ref speed..
It also means that if you are a reasonable pilot but are landing at an airfield with limiting public transport runway length that what the rules actually do is limit the amount of fuel you can arrive with. Which is not great..
I tried to find out the origin of the change in factor from 1.43 to 1.67 when I was on a postholders course and nobody seemed to know. Its interesting that a number of operators (BA, Virgin etc) seem to use a factor of 1.5... lets play spot the level playing field..