The problem here is that "tritone" has varying definitions, including at least: half an octave, three whole tones, two minor thirds, and that particular in JI, that leaves things a bit wooly to say the least.

It appears fairly conventional to say at least that the lesser undecimal tritone is the 11th harmonic, which corresponds to a ratio (folded into the octave) of 11/8, but other defintions (64/45 or 45/32 for example) also exist.