The most accurate is to use the method Eskimo lays-out, and base your future predictions on your past performance (SAC-rates). Much like you would estimate the ’range’ you might get out of a tank of gas in your car based upon past MPG calculations. There will always be some variables even during a single dive, but I find my SAC rates run very consistent, which makes using my past SAC rates as a predictor for what I can do on upcoming dives pretty accurate. No reason to repeat the formula because Eskimo provided it above.