For the ranges considered in F4, you never get near the normal values for best specific range or max range, simply due to the short legs. Mission planning data is optimized for legs of 250 nm or more. Shorter range missions than that, the climb and descent end up using more fuel than you would otherwise save by climbing to the most fuel efficient altitude.
Graphic calculation of specific range from the tables is a trivial piece of maths, but factor in leg range, climb and descent, and you end up relying on multiple iterations, looking for a min value of total fuel consumed. Its easy to say “this is the most fuel efficient altitude you can fly at for this GW and DI”. Its a little harder to say “this cruise altitude for this mission will result in minimum fuel consumed”. Unless the leg is more than 250 miles, in which case, its easy.