The diversion routes are published on the FP, they are direct.
It is not necessarily ALL about experience because experience will drive you towards the everything normal scenario. It is a lot about common sense and assuming the worst case scenario. How many times in the past 15 years did my fuel decision save me......once. 1hr overburn and not one second of that in a hold!! landed 300kg above final reserve.
I have experience, that told me there would likely be no problems, it was common sense told me to take a whole bunch of gas!