GL...think you made one mistake in your comparison.
According to the official F2 page: "The FX V has a tail width of an impressive 83 cm measured 30 cm from the tail. " So it is not 78.5 as you wrote.
in this drawing process, as I've described, there is a not trascurable marginal error (I think about 1-2 cm particulary in horizontal measures) due orginal photos perspective distorsion.
Anyway I've taken care to minimize this error as much as possible (vertical measures should be quite correct, instead).
About FX-100 v tail at O.F.O. lenght (30,48 cm), I've found it measures merely 79,1 cm; I could be wrong, but even if we assume a marginal error of 2 cm, FX-100 v tail should be 81 cm and not 83.