Would suggest something along the following lines:

This calculator assumes the forklift is empty, or if loaded it is fitted with hard rubber tyres.

For a loaded forklift fitted with cushion or pneumatic tyres the measured clearance on the level may be reduced by 0.4 - 0.6 inches. It is suggested that for these forklifts you reduce the clearance by 0.6 in before inputting it into the calculator. For example if the clearance when empty is 4 inches input 3.4 inches