Fortunately, Deliiffa, as Whatsnext already said, this whole business is just not that precise. If you do your WBTB after around five or six hours, you will probably catch a REM period. If you don't, then take Whatsnext's good advice and do a little experimenting until you find the best general time to wake up... and even then it will still be a general time, because your sleep cycle might tend to vary from night to night, depending on any number of circumstances (or even nothing at all).

You might also consider doing without an alarm at all. Alarms tend to startle you from sleep and wake you up too thoroughly, which can make it harder to hang onto your dreamy feeling during WBTB. If there's a chance you can experiment with waking naturally after 5 or 6 hrs, you might try that as well.