Help with timing sleep cycles

Hi all

For a Wbtb I like to sleep for 6 hours then wake for 30-40 mins then go back to sleep. This is based on what's usually written about it also on the idea of a sleep cycle being 90 mins typically. I wonder though, surely this is 90 mins from when you actually get to sleep...
My main question tho regards when I've been woken up by my kids after any given time period, 4 hours for instance...should I then set my alarm for 2 hours to complete the 6 hour (4x90 min cycles) (the time I had expected to get up at)or set it for 90 mins/ 3hrs, for 1/2 cycles from then?

Thanks

P