To answer your question more directly, there doesn't seem to be any particular "formula" from which the ENY values for a particular aircraft are calculated or derived, except that (obviously) GENERALLY! the more capable the aircraft the lower the assigned ENY value.
Again obviously, unless two aircraft are grossly mismatched, people can probably argue endlessly why the relative ENY between any two aircraft are "out of whack". Take the two aircraft you mention - the 51D is generally a more dangerous fighter (in average hands) while the Jug can carry enough ordinance to take down most of a town all by itself.
Generally and by and large, the respective assignments seem to me to be generally "about right".