Saliva ovulation testers - Partial fern or full fern etc?

For those who have used a Saliva tester, is there a pattern in general to look for, I mean I know that every woman is different etc. But was wondering if it was like cm and build up to Ovulation like that, like do you get a few days of partial fern before getting full fern? and the day you get the full fern pattern is that when you are ovulating or is it like OPK's and ovulation is about to happen in thenext day or so?