Where do you get those figures from? If you read the specs of harddrives, they are more like 5V@300mA and 12V@500mA. mA is milliAmps, which is 1000th of 1 Amp, so 500mA is 0.5A. I think the figures are more like for rewriters. My LiteOn says 1.5A for both voltages.
grundyv2, here's some formulas for u:-
watts=current * voltage, where current is in amps (not milli-amps)
convert mA to A:
So for your 10 Benqs asumming they use 1.5A@12V & 5V, this will be:
1.5*12 + 1.5*5 = 25.5W
25.5*10 = 255W
This is only the drives, which excludes every thing else!
As for PSU watt rating, I wonder if it applies to total watt or total for each power rail. If, for each then you only need to calculate for the higher consumption only, i.e 12V.