I haven't got one right yet either, despite having a spreadsheet to model it all!
In order to rectify that, I have updated said spreadsheet with a polynomial curve based on the tyre degredation figures the last time this combination of soft and hard was used.
This scientific method says tells me that the best way to do this race is 3 stops, spending as little time as possible on the prime tyres. Anyone pitting too early may well find themselves having to run way too far on hard tyres which look at least 1.5 secs a lap slower initially. Even options that have done 15 laps are likely to be as fast as brand new primes, although by then the options are going off rapidly.
My guess is then, 3 stops would be as follows
1) if I used all three options in qually: laps 13, 26, between 39 and 42
2) if I have a spare set of new options: laps 13, 26 and 45
Science will also tell me if it is going to rain - although my spreadsheet will not like getting wet so it may not be any use
.I will be using raintoday.co.uk to see what is coming!
Gonna be some weird weather then 
.
but if not, then, intermediates weren't invented back then. 
