Here's the major dictionary (Liddell & Scott) entry for the adjective ἐπόγδοος:
ἐπόγδοος, ον, 1 1/8, Pl.Ti.36a, 36b ; ἐ. λόγος the ratio
of 9 : 8, Plu.2.367f; ἐ. [τόκος] interest at the rate of 1/8 of the principal, i.e. 12 1/2%, D.50.17 : neuter as Substantive, whole tone in Music, Philol.6, Hsch.
Someone schooled in music theory may be able to elucidate further.
The terms spelled in Greek: διατεσσάρων, διαπέντε, διαπασῶν, and ἐπογδόων