I understand and applaud your wish to become more efficient in the water. It's a good goal. But you do recognize that your air consumption while diving will and SHOULD never approach your resting surface SAC rate, right? It would be far more realistic to try to identify a surface activity with a similar energy expenditure to the demands of a well-executed dive, and compare to that. But I have no idea how you would determine what the surface activity comparable to a well-performed dive in your local conditions would be.
I think the best you can do is to figure your SAC rate for a fairly reproducible dive, if you have one. For example, we have a very commonly dived local site, where you can describe a pretty typical tour. If you do that dive repeatedly, you can compare the trends in air consumption over time, or with gear or technique changes. Since no two dives are precisely the same, you will need a good set of data points to have reasonably meaningful information, but if you dive frequently, that's not too hard to do. I dive these local sites often, so it was easy for me to see that one of my first training dives using a reel caused a 50% increase in my SAC rate!
I think the best you can do is to figure your SAC rate for a fairly reproducible dive, if you have one. For example, we have a very commonly dived local site, where you can describe a pretty typical tour. If you do that dive repeatedly, you can compare the trends in air consumption over time, or with gear or technique changes. Since no two dives are precisely the same, you will need a good set of data points to have reasonably meaningful information, but if you dive frequently, that's not too hard to do. I dive these local sites often, so it was easy for me to see that one of my first training dives using a reel caused a 50% increase in my SAC rate!