If you think of a dominant 7th chord as four notes in a 4:5:6:7 resonance, then it seems the 7/5 interval is the "correct" tritone in this context. But a proper tritone substitution means turning the 7/5 into a 10/7 or a 10/7 into a 7/5, which means one or the other of the chords will be out of tune with itself.

Alternatively, one could just hold one note fixed and move the other three, so you end up with both chords being self-consistent. That seems more mathematically satisfying at least.

Ultimately, though, I suppose the only real standard of correctness in music is whatever you can get away with.