Dream Recall, Timing, and Sleeping In

Alright, so, let's take it from the top. I'm 18 and in my senior (12th) year of high school. Good grades, fairly low stress (at the moment, although I have rare 'panic attacks'). I take a long-release B-100 complex at around lunch or dinner times or, failing that, breakfast. I keep a dream journal, if disorganized, whenever I remember more than a few faint scraps of a dream- I've got a dedicated journal by my bed, now. But, despite my best efforts, something is nuking my dream recall, and I don't think it's a case of wiggling around too much when I wake up.

You see, consistently, I've noticed that I'll recall dreams on weekends. Why weekends? Because on weekends, I can sleep in, which not only gives me more sleep, but allows me one other special concession: random interruption. Whether it's some shift downstairs or someone walking into my room to tell me something, if I'm interrupted by something, I'll remember a dream- and often somewhat vividly. "How might I capitalize on that?" I asked myself. "Surely, the reason must be that I'm being interrupted during a REM cycle!"

So, here's where my personal experimental aspect of things comes in. On the average weekday, I go to bed at 11:30 PM to wake up at 7:00; roughly 7 1/2 hours of sleep, ideally. That gives me enough time, at 7:00, to get ready for school and out the door. Knowing this, I hatched a plan: by using a set of three alarms, I'd base a system of interruptions on the 90-minute REM rule (which, in retrospect, I'm pretty sure I didn't do nearly enough research into).

The alarms were as follows:

1. 4:00, which gave me 4 1/2 hours before interruptions started;
2. 5:30, 90 minutes later, for a second attempt at interruptions later in the night; and
3. 7:00, 90 minutes later again, to try one more interruption and get me ready to go.

I gave this a whirl for a Thursday and wound up with absolutely nothing. EDIT: (My alarms are somewhat startling, which probably contributes to the issue. Phone alarms, man.) So now come the questions:

- Did I manage to wake myself up at the exact end of REM cycles instead, rather than the middle, due to a grievous miscalculation?
- If I shift the times, what direction should they be shifted and how much? I tentatively have them set a half hour backwards.
- Should the gaps be made smaller between the latter awakenings? Potentially 45 minutes, instead?
- Should I get more 'preliminary' sleep (that is, before the first alarm) before I do anything else?

If anybody can provide some advice, or, ideally, give an estimate of where my REM cycles might lay between 11:30 PM to 7:00-- or, better yet, provide me some way of gauging said cycles independently (as they likely vary from person to person), any or all of these would be deeply appreciated. Thanks for reading this far.