Setting ride height on E Types is not an exact science. There are a  number of factors which come into play, including the condition and specification of torsion bars and bushings.

The factory setting tool is designed to be used between the upper and lower mounting points of the front shock absorber. For standard E Types, where the torsion bars etc are in perfect condition, the manual calls for a distance of 17 13/16" between the two shock absorber mounting points. When heavy duty torsion bars are installed, the additional strength of the torsion bars means that a distance of more like 16 1/4" will result in the correct ride height.