Well, this question (about where to calculate it) immediately points out one of the weaknesses of the number. The best way to calculate SAC rate is to get to a known depth (say 33 feet) and swim at a steady rate for five minutes, and then use those numbers to figure SAC. The problem with this is that this number isn't very useful for planning a whole dive, as it's rare that you will swim steadily through a whole dive (cave diving excepted!)
For real purposes of tracking and planning, I've found using the average depth for the dive (from my computer/gauge) and the total gas consumption and time, gives me a number that incorporates the times I was swimming, the times I was sitting and looking at something, and the reduced gas use on ascent. If I were planning a dive quite different from my ordinary reef-swimming "look at the fishies" dive, I'd have to adjust my expected gas consumption accordingly.
As you accumulate dives and do these calculations over and over again, you'll find trends. I know, for example, that my gas consumption goes up when you put a reel in my hand. I know I use less gas on a warm water drift dive. I know what doubles versus singes does to it. This just comes out of having a bunch of data points and spotting trends.