I just measured my 35 gallon HEXagon tank: it's glass panels are 11.5" wide x 25" tall.
Measuring across the top - from flat side to opposite flat side - I have 20". This is where the legs of my light fixture rest, so this is the measurement that is more useful. Not the distance from angled pointy joint where the glass panes meet. (23")
Not an exact answer, but yes, seems that calculation is off.