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.
You have something wrong if your card is drawing 40 watts idle. They should be <15 watts idle with the screen on and they drop to <5 watts idle with the screen off.
