I know that everyone says that sleep cycles last 90 minutes. No one mentions how the change in REM period durations affects this. Would someone help me figure out the best time to set an alarm for WBTB?

Lets say I fall asleep at 12 midnight, and wake up at 8. My "alarm" is a cell phone set to vibrate stuffed in my sock. It has 3 slots for an alarm. I am using two of them, set to 3:00 and 6:00 respectively. I have heard anywhere from 3-8 hours is effective, but what times do you find to be the most effective?