I've thought about that as well... one way to mitigate the risk would be to add a warning to the warning list as long as the calculated takeoff times and fuel amounts haven't been manually updated.
Another even more flexible option would be to just copy the weights and add a small button to calculate the takeoff time and fuel amount, maybe even with a mini dialogue:
+--------------------------------------------------------+
| Calculated takeoff time from previous flight:
| [ ] Takeoff + Flight time + [ xxx ] Minutes
| [ ] round to the next [ 5 / 10 / 15 / 30 ] Minutes
|
| Calculated remaining fuel from previous flight:
| [ ] Use default fuel
| [ ] Use last fuel - required fuel including reserves
| [ ] Use last fuel - required fuel with half reserves
| [ ] Use last fuel - required fuel without reserves
+-----------------------------------------------------------------