Up2code:
Are you sure the fixtures are 4x40watt? Or did you take the info from the ballast?
Reason I mention it is most T-8, 4 lamp ballasts are under 1 amp @ 120 volts (<120 watts) which will bring your numbers down substantially.
Electronic ballasts have a 'THD' (total harmonic distortion) percentage on the label, which you will need to do the calcs.
Steve Fehr is the EE on site, & I'm sure he has a equation..
A basic 'rule of thumb', IMHO would be #8 neutral with your numbers.
