Applying yourself while doing calculations should really suffice. Do not get distracted by other things around you. I personally don't think that you need calculation drills. But, if that gives you more confidence, then go ahead.

Are you currently doing the calculations in your head? If so that may be the problem. If you do not usually do calculations in your head on a regular basis then you should not try to do them that way on the GMAT.

Remember that there are not that many actual calculations to be made on the test. You have to set up equations, ratios, formulas, and know your number properties but there are not really that many calculations. It is worth the time to do these calculations calmly and not in your head.

Missing questions on the GMAT is a fact and not a tragedy, what is a tragedy is missing a question that you know how to do, take the time to set up and then miss after 2-plus minutes do to a calculation error. Or a question that takes you another minute or two to realize just where your error was. Better to take a few extra seconds and make your calculations calmly and on your note board and as Papgust says don't get distracted - focus when calculating.

thanks for your help.