Ren, IMO, if you found a way to beat the system, and it sounds like you have to me albeit DAluminum's caveat that is is magnetic, then your fine. I'm just jealous I didn't find that when I was taking the lighting tour in my local Wall-Mart.
Oh, as you probably know, the other thing to consider is the voltage across the bulb terms. The total wattage is what is important, not just the current. But I'm sure you know this since you worked in electronics some. Just for scale, a 24in VHO requires 75 Watts while a 24in NO requires 20 Watts (but NO can be driven at VHO Watts with electronic, but not magnetic ballasts).
EDIT: and TWoods, I agree depth makes a difference, but in defense of WPG, that is why there is a broad range often quoted. So for deeper tanks, you should lean on the high side. For shallower tanks, you can lean on the lower side.
Congrats!
kris