Originally Posted by
rainbowflower
the temperature shift is based on the latest THREE temperatures being higher than the previous 6 temperatures - it is not based on it being over a specific temperature as your baseline (or coverline) can vary each cycle meaning you might have temperatures on average slightly higher/lower in different cycles
the white spot on your chart indicates that the temperature might be inaccurate which could affect the ovulation prediction - this might mean you took it earlier/later than normal, or were poorly... even 10 mins different can affect the temperature by quite a bit
in your case, the ovulation day might change a bit if you have another very high temperature as your next temperature input, and you might have ovulated on CD19 instead of CD17. Usually you ovulate on the last day of EWCM so this would make sense there.
I've found it's ovulation prediction to be fairly accurate, although my chart for my son told me I ovulated on CD21, and the dating scan brought my dates forwards a day so I either ovulated on CD20 or conceived the moment the egg popped out ;)
then again, scans themselves are not the most accurate either.
It would be unusual for FF to be out by more than a couple of days.
Sorry that's a rather lengthy reply!