Understanding take off performance is the key. The idea of take off performance is not to use the least amount of runway and get airborne! Many factors are taken into account. Runway length - Yes, but also obstacles close in, terrain further out, then all these again with one engine! The idea is then to take the worst case scenario, with that days weather conditions. Finally, if your take-off weight is less than the calculation says is the maximum, you can then flex/ derate or use assumed temp etc. to simulate that weight.
The point you leave the runway is irrelevent, what really matters is that before V1 you can stop and after V1 (as you must go) you will clear all obstacles and terrain and have enough runway to get to the speed to achieve that.
Finally dont forget, an extra 50 people weigh about 5 tonnes. Not very much change for an A330 I would guess (I drive little 73's) You could take several extras tonnes of fuel if your first alternate is out!
Many variables to consider.....