There is a lot of variance in sleep cycles between individuals, between nights, between different types of sleep. Saying 'a sleep cycle is 90 minutes' is like saying 'The left side of the brain is for logic'. There is a rough element of truth there, but nothing to hang your hat on.

Source: once worked as an overnight sleep lab technician, collaborated with sleep specialists in next job working for the lab equipment maker.

Also, a timer with 90 minute steps inherently includes 45-minute steps. If you do desperately want the in-between steps, it's a trivial mental calculation on the four steps you do get given at the site.