BOAC, you might settle on a simple rule of thumb – type dependant, but it wouldn’t work everywhere; try the first third at LCY !! (Don’t). The important aspects are in the details, 'a and r'.
Another disadvantage is that if rules of thumb (RofT) are used as a ‘sop’, then this can reduce the perceived need for knowledge of landing performance and its dependencies, variables, etc, and reduce the crew's ability to calculate the actual landing distance required, i.e. not understanding the circumstances when the RofT must be ditched.
Conversely, with exemplary knowledge, and due consideration of the variables, the landing distance calculated can be checked with a RofT.
Pilots need to be encouraged to think about the problems and then check afterwards, a similar idea in some posts above about the use of reverse; if you are going to select it, then use it all.