In response to the orginal question.....

Take the original machine rating and calculate the load moment including the 'x' dimension calculated from the centre of the wheel to front face of forks. So 3000 (700 + X dimension). This provides the load moment trying to lift the back wheels off the ground measured from the front axle.

Then, at the other end of the machine, take the unladen steer axle weight and multiply by the wheelbase. (This gives you a load moment of the counterbalance, measured from the centre of the front axle)

Now the rear load moment must be greater than the front load moment (or else you have a huge problem) however the ratio of one to the other is important and must be maintained at all times. (this is also known as the safety factor and is a good measure of machine stability)

Keeping that same safety ratio within the calculation, resubmit the front end-calculation inserting 700mm as the load centre but keeping the capacity as a variable (say 'L') instead of 3000. The new calculation will then put a figure against L and will retain the original safety factor.

Hope that helps