Great question!
For many tendons it is obvious what position will relax it, for example in the Achilles you should have passive plantar flexion.
But with tib post it is not so obvious.
I use a little ankle inversion and passive plantar flexion to get the best flow.
The good news is that flow in tib post when it has tenosynovitis is present regardless of the patient position, so it is fairly forgiving.
But for the best posture try a little ankle inversion and dorsiflexion.
Steve