How do I time WBTB correctly?

I do not fully understand what time I am supposed to wake myself up for a WBTB. According to the DV tutorial on WBTB, one must wake up before a REM cycle is over. Each REM cycle lasts 90 minutes long, so I should wake up part way through one of those REM cycles, correct? However, even though I try to be consistent with the time I go to bed each day, I find that I fall asleep at different times each day, sometimes it takes 15 minutes, and sometimes it takes longer than 1 hour and a half. So that makes it very difficult for me to time my WBTB accordingly.
Any help would be appreciated.

The time you wake is not as important as the time you return to sleep. You want to return to sleep when you know you should be entering a long, stable REM cycle. You can adjust the time you awake, or the length of time you stay awake. Take a few nights trying to pin down when your best dreams happen.

Sometimes it is easier not to overthink it. Our bodies are sensitive to all sorts of stuff, and are not 100% predictable. Just guess, and adjust accordingly the next night

In two of my lucids, I had woken up around 2/3 A.M, and stayed awake (not intentionally; tried going back to sleep) for several hours. When I finally did fall asleep, in both cases I found myself in sleep paralysis, which enabled me to enter the lucid dreams. Perhaps I should try waking up at around 2 A.M. for my WBTB's.