Alex
If you wanted to run a ten lap stint and push hard, fuel for 12 laps, after ten laps you'll have used your fuel and pull in to the pits anyway. As you can only change push level when you are present this technique will work fine.
Personally I wish you could pre-manage push levels, ie lap 1 - 9 take it easy, lap 10 push hard (before pitstop), lap 11 - 19 take it easy... But I'm not even going there when so much basic stuff needs tweeking first. However, point being, you pit when you've used your fuel, and you're locked into one push level when you're not live, so do the maths.
Allowing pre-management of push levels would take away from the live management aspect of the game.
That's why the mobile app is up next in the pipeline, so that live managing doesn't require a PC when one isn't available.