Salaam.
The derivatives of the root VST (T here represents the sixteenth letter of the Arabic Alphabet) are found in just five places in Al Qur'an:
2:143, 2:238, 5:89, 68:28, & 100:5.
Justly balnced, Middle or the best and the most excellent, average, a degree better, and, (as a verb, meaning:) charged right amidst, (respectively). The underlined, is presently the subject of our discussion.
According to Abdulla Yusuf Ali, exegetes have rendered it in both ways, some preferring the one over the the other.
In all humbleness, as far as I have seen, when juxtaposed with Salat al Fajr and Salath Al "Isha (both specified in 24:58), both of which are Possessive clauses, and since Al Salat Al Wusta is an Adjectival clause, it is about the quality and not about the mathematical or physical position.
Regards,
A. Ismail Sait.