You also have to remember that as the aircraft rises it will be consuming fuel reducing the thrust that is required to maintain a constant upward acceleration. Also as you do this to maintain a steady acceleration you must reduce the thrust to maintain the same thrust to weight ratio.
Here's some help for you. The weight of an empty Harrier is 22,950lbs and it provides 23,800 lbs of thrust from its engines Pegasus Turbofan engine. In order to find the amount of fuel needed to take off you now need to find the amount of fuel consumed in order to achieve >97% of it's maximum...