If you're measuring the card use, you're just concerned about the delta between load and idle, not the total usage.
The MH/watt numbers would be skewed if you're only measuring delta.
OP should test with no graphics card idle in order to establish a baseline.
If he's got two 7970s, he can measure the system idling with 1 card and 2 cards and that will tell him his idle use. I didn't think the point of the thread was to measure the MW/watt for the system, only the card.