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.
Measuring delta doesnt measure the MH/W of the card accurately either. If you measure idle power and then add to delta, that would give you the true power in order to figure out the card MH/W. The idle power of these cards aren't exactly negligible compared to the load power. Mine sucks up ~40 watts idle.
Also, related thread on the subject:
https://bitcointalk.org/index.php?topic=57410.0