How accurate is the PowerPrep GRE score?

Very accurate. There are only five of them, so treat them like a cherished set of glassware that you only bring out when it really matters. These are your most accurate score predictors, since they use retired official questions and replicate the GRE's section-adaptive scoring. Use them when you're ready, because they're your three chances for the closest thing to a real GRE score you can get without taking the actual test. Usually that means after some study and after learning basic strategy from third-party exams, and only when you can block out time and space to take them under realistic conditions (timed, no interruptions or pausing, in one sitting).