Suggested

Use user chosen lap not estimated

Yes. IMO you should first set the lap you want to pit and this is what the game would use as the trigger to pit (turn off lthe low fuel pit trigger).

The game would automatically populate the estimated fuel using a conservative calculation... in other words, give you a little too much to complete the desired laps even at PL5.

Then, very much like no refuelling, it is up to the player to trim back on the fuel load to a lower but more risky value.

Get it just right and your car will be light and fast, too much fuel and the car will be slower because of excess weight, too little and you risk running out.

If you're on line and you think you've under fueled you can always adjust fuel for the following stint and pit earlier than your pre-race strategy.

This would also allow people in refuelling leagues more flexibility to adjust their strategy during the race to counter the strategies of their competitors.