A simple system was suggested to me a while ago. Winner of any TT gets one point down to however many places (say 120?).
When entering other races, best 10 results from the beginning of previous season to entry date are taken for a points total, lowest total gets in first. No need for risk taking in the traffic on some of the "fast" courses. A simple start, but a good one, and fairer as it relates peer placings. Not perfect, but seems to be a bit more reasonable than simply time which whilst an absolute is subject to far more variables with weather, traffic, course, etc. I hasten to add it's not my idea, personally I'm not bothered as I can't see the point of TT's week in and week out (something unique to this island), but whatever floats your boat....