What you can do is set it to 10A and take one pin out of the PSU header plug at a time and put the meter in between it, then read off the current. Then from that you work out how many watts are being consumed for each voltage (VxA) then add them all together.
Do it over some time while accessing internal hard drives etc so you get a maximum figure.
Ok this will take a while. Alternatively you can just stop worrying and just get the damn ATX Psu cos I dont think that a Falcon is gonna consume more than the smallest ATX supply you can get your hands on (brand new tend to be at least 400W these days).
