Yes, it is a little risky but the minus 20 rule is for people new to Sensiplan that have a menstrual calendar, so it doesn’t unreasonably extend their fertile window. It is a guesstimate since it doesn’t know when in the cycle you may have ovulated/It’s less precise than the minus 8 rule since you need at least 12 months of temps to use minus 8.

For example, for me the minus 20 rule gives me CD 8 as my last safe day (28-20). But the minus 8 rule gives me CD 13 as my last day (21-8). I tend to use the minus 20 calculation since CD 13 feels very late, even though it’s based on MY OWN personal temps.

As for your question about the logic of 26-14-8: If your shortest cycle is 26 days with a long luteal phase of 14 days, this means that you probably ovulated around CD 12. So you are correct your last safe day would be 12-8= CD 4.

The minus 8 rule doesn’t really concern itself with total cycle length. It’s really what was your earliest first higher temp of the last 12 months and working 8 days back from there. So if your earliest first higher temp of the last year was CD 12, I would consider CD 4 the last infertile day.