Depending on what your mechanism is you make different adjustments for each model.
Lets assume you have the old system with the float on a long metal dowel. The curvature of the dowel has everything to do with the amount of water in the tank. Of course you do not want too much as the water will continually run down the drain hole in the tank and continousally run.
The curve should be in the direction of an umbrella extended. So, to get more water in the tank, straighten the dowel more carefully to extend the float out more and this will allow more water.
Some models have a adjusting screw where the dowel pivots on the hinge that will do the same thing easier.
Experiment with this until you reach the water level you are seeking. It takes time and many experimental flushes.