If that's the case, then the in-flight spawns should just have a fuel deduction on arrival based on the duration required at cruise to reach the box. I have little doubt that, once in proximity, they'd very much like to increase throttle.
Of course, this would also mean that the return flight fuel would need to be deducted as well once they exit... Or, perhaps an a priori reduction from their supply. This would leave them in an appropriate place, decision-wise: go all out and risk exit or fly cool and run a greater risk of interception.
And, of course, none of this means anything for the main arena... But for scenario, realism is worthwhile.