Well, since I just learned all about this SAC rate thing... the advice I got was to track your SAC rate over multiple dives and just watch what happens on different dives to give you an idea what to expect. If you do 20 cold quarries dives, you can look at the average and make a good estimation of your air usage for the next cold quarry dive. If you then go to Bonaire and do a warm dive, you can look at how that compares to your "average" dive and be able to plan within a certain degree of error for the rest of your trip, calculating each dive and adjusting as needed. I was told that it's a process and you just need to start tracking to see how you, individually, react to different situations.
Rachel