We’re a US Carrier and subject to 120-42A regarding ETOPS – I suspect that you have a similar document. The answer is sort of both. If you are using a tail number specific fuel figure you just run the entire flight plan with it – so it affects both. If you are going with the straight 5% it only affects the diversion leg. The curious thing is that if you have a 3 % above normal fuel burn which is lower than the 5% - you can end up carrying more fuel because you have to account for + 3% above the book value on the flight to ETP and you save only 2% on the diversion leg – these figures obviously can vary due to a lot of circumstances. Of course if your Fuel Mileage is book value or better, you are always going to save.
Actually 120-42A doesn’t help much when you are try to figure this out – but this is how we do it – and I believe others.