Decrease the power/gas to your heating device. Evaporation loss will be dependent on the amount of energy you put into your kettle/wort. The temperature will pretty much remain constant during the boil which means if you put more energy in, the result will be more water loss as energy will be carried off in the form of vapor. This is less efficient and wastes energy.
So you can't calculate this due to the energy dependence. Empirical observation, trial/error, and experience will give you an idea of how much water you lose. Try marking your settings; also you will get a feel for the type of boil. (i.e. rolling boil, simmer, somewhere in-between)
Also there has been debate regarding - a hard boil will help drive off DMS and affect hop utilization. I haven't seen data for this and have made excellent AG batches uses a less vigorous boil. From a chemical standpoint, the hop utilization is much more dependent on temperature (which is constant) and density/gravity of the wort.