PTA is a formula that I recommend remembering as one of the basic formulas needed. It is used extensively in Fixed Price Incentive Fee Contracts and really, these contracts cannot be done without it. The thing about the PMI test is this, you should be gathering and remembering information from many sources, not just the PMBOK guide. Again, I believe this is one that could pop up during the test and you should be able to remember it.
PMPrepcast has a formulas course, which I recommend here:
www.project-management-formulas.com/index.php/product
It covers 47 formulas that could appear on the test.
Though I was able to put to memory all 47 formulas, the test only gave me a handful of questions; the key was that I was prepared.