Three Peaks Fell Race

[Joe]

Orienteers usually reckon that 100m on the flat is equal to 10m of extra climb (or vice-versa) OR each extra mile = 500ft of climbing. Using this as a basis I looked at a range of races and calculated a severity rating by the formula ft climb/500+distance.

This is just a tweak of Naismaith's time honoured formula:

Allow 1 hour for every 3 miles (5 km) forward, plus ½ hour for every 1000 feet (300 metres) of ascent.

Thus 5000m distance is equivalent to 600m ascent, hence 1 mile is equivalent to 634 feet, and so 1000 feet is equivalent to 1.6 miles. For simplicity I quoted 1000 ft == 1.5 miles.
In practice I've found this to be a good judge of how long it will take me to run any given route, ignoring rough terrain and any ludicrously steep descents.
For fell running this formula is obviously superior to yours due to the extra importance it places on ascent

Anyone got a better formula that can be demonstrated to rank races more accurately than this?

This sort of formula does very well at giving an overall sense of how "big" a race is, so it's very good for ranking S/M/L, but it doesn't do anything for the A/B/C debate.