I always well-pad my bingo fuel to allow for the unexpected at the alternate, as not only you but every other Tom Dick and Harry (and the odd Harriet) will probably be heading to the same place, on top of the scheduled inbound traffic to said alternate. This of course depends on the actual circumstances however, and how the CFP divert fuel is computed.
The box at least gives you an idea of how long you can hang around, but I never take its computations as written in stone.