This is my third month using the monitor, and I think the monitor is still "getting to know me" and my surge day tends to vary. The first month I used it I started using the sticks on day 6 and got a + opk on cd16 and "peak" on cd17. Last month, the monitor started asking for sticks on cd 9, and I got my "peak" on day 13 - the same day as my positive opk. I just starting using the monitor today (cd8) - I guess we'll see what kind of month this is.
FYI - after you get your first peak, there is no need to keep using sticks as it then goes into automatic mode regardless what your sticks say. I learned a lot on the b.abycenter boards - there's one dedicated to this monitor.