It mainly depends on how long your sleep cycle is. As you sleep, you go through the different stages of sleep, from stage 1 to stage 4, followed by REM sleep, which repeats every 60-120 minutes (90 minutes on average). However, it varies for each person.



As you can see, the amount of REM sleep (the red lines at stage 1) get longer after each sleep cycle, which is why waiting 4-6 hour is best, since after 3-4 cycles, your REM stage is longer than in the first stage. However, it's hard to nail down a specific time, as like I said earlier, sleep cycles are different for each person.

But like ThreeCat said, it just take practice. Start with 4.5 hours, and if that doesn't work, try tweaking it a little.