wow wow I can answer this question....
If you read the fundamentals of better diving, it says you should roughly know what you have in your tanks at all times, the more advanced diver you become the closer you should know to what gas is remaining.
TEXT BOOK, well not quite
P.S. If you have had to abort the dive early you should know that you have more than enough gas to finish. Oh and if you are completing deco there are appropriate mixes that should be used instead of back gas to speed deco. That last one point was an educated guess.
OPNE DAY I WILL READ THE QUESTION BEFORE I WRITE AN ANSWER, SORRY JUST HAD LONG DRIVE BACK AFTER HARD DAY!!!!!!