You have started off with a basic error. Bulb wattage is irrelevant.
Instead, look at the ballasts. The current marked on the ballast is the figure you use for your load calculations. Also look at the ballast for the 'power factor.' While 'power factor' is technically different from 'harmonic distortion,' it still seems to work out that ballasts with high power factors (greater than .93) typically do not have harmonic issues - even if the fixtures have lables warning of the risk.
The other 'ace in the hole' that you have is that the NEC understates the ampacity of #14, #12, and #10 wire. This means that your neutral is already slightly oversized ... I'd use #10 on a 30A circuit without fear.
For a final check, hook up one string of lights. Measure your amp draw (on the neutral) two ways: with a 'normal' meter, and with a "True RMS" meter. If the readings differ by more than 15%, you need to take harmonics into consideration. Less than 12%, you can disregard the issue.
