When preforming a WBTB with the intent of following it up with a WILD you want to wake up between REM cycles, preferable right at the end of one. This is where the 5-6 hours after you fall asleep comes in. It's also 5-6 hours from when you fall asleep, so if you fall asleep at 2, set an alarm/wake up at 7 or 8. If you fall asleep at 6 set an alarm for 11/12, and so on. However everyone's sleep pattern is different so you'll have to experiment with times until you find the one that works best for you. The only practical way to find your REM cycles is to experiment, trial and error. You could hook yourself up to a brain scan to detect dreams or when they end, but that's just silly