Well, unless I forgotten my math, Watts/volts=amps. One 40 watt T-12 at 120 volts = 0,33 amps times two .66 amps.
Do the same for 32 watts T-12.
You can find the charts or calculator's online, type in "watts to amps" or amps to volts. Most of these site will many difference calculators to do the math or list the formulas.
That's it in a nut shell.