Measuring solar yield approaching winter solstice

You won't get much time in the day with the sun over 15 degrees altitude, which gets it above the worst of the atmospherics.

At 30N, and Declination 23S, Altitude at high noon is 37 degrees.

(Lat is Zenith Distance - Dec, so at 30N, Dec 23S: ZD must be 53 and Alt (90-ZD) 37 ) So your tilt angle should be 53 at high noon and higher than 53 earlier and later when the sun is lower. So your "optimum" tilt for the most AH haul per day must be more like 63 ? Flat is going to be a comparative bust.