From the what it is worth dept. Most ramps are built on a 10% grade or the vertical height from ground level to the top of the entryway (dock floor) x 10 will give you the horizontal length of the ramp required.
Using the maximum gradeability from a spec sheet can be dangerous, as most of these specs are measured in ideal situation in a testing lab (controlled conditions) - new truck with less than 100 hours, new tires, rough brushed concrete surface, truck travel speed at 1 mph or even at the point of stall (to get a bigger numbers for the buying public); the engine and transmission in perfect running condition (new plugs, etc etc. Also, gradeability #s will vary widely from brand to brand, except Mistubishi - Cat & Yale-Hyster (same units different decals & paint color).
Most truck can achieve a 10% ramp either empty or loaded. Generally speaking the empty gradeability that is published is becasue of drive tire wheel slippage, yes there are a few exceptions.