Thanks villas,
Your right that would be a more straight forward path. At least then your focus will be just on the extra fuel burnt and not on potential pitfalls of not monitoring a fuel imbalance correctly.
Looked through a few potential scenarios the other day inflight. Think you would be having a bad day to have it top of climb at FL390 on a long flight with little extra fuel. Although knowing the sim I'm sure with other events thrown in would make an interesting sim.
Thanks for all your help.