Would it not have been a wise idea to have a "standard" W & B calculation that allowed for full to empty tanks and two pax weighing between say 70 to 100 kgs each, in the front seats. At the same time a performance calculation could prepared that would allow a departure from a 1,200 M runway with a 2% upslope 700' altitude at ISA plus 15 with a QNH of 990 hPa and say 5 kts tailwind. Then providing none of these limits are bust, you do nothing.
PM