Sorry Gavin
It should be 18/25, not 18/27.
one thing that you should keep in mind is, the smaller the is the less tyre wear.
at moorebank in 30 min finals start with 62.5mm 40shore rear, when finish it measure to about 59.5mm. when I start with 60mm by the time I finish it measure to 58mm, so you see the wear rate is constainly changing, not to mention if you are using different compound for f/r tyres it throws in more variable.
I think ODR is important but the driving style has alot to do with it as well, especially in endurance race it has to smooth to save tyres and fuel mileage also not to stress the engine. Happy Racing