spruncegoose,
Its up to the airline to decide what slope it wants to use, we generally go for the easy option of taking the runway end elevations divided by the runway length. But in certain cases where we know that the runway dips in the middle and that the slope on the accelerate go portion is greater than the average, i.e Manila, we just use the maximum slope.
The penalty for doing this on a B747 is around 3000 kgs, but is a lot less costly than putting the aircraft off the end of the runway.
Cheers
Mutt.