Paddington, there is no "absolute" value. It just needs to be higher than it is pre-ovulation. But some people have generally low temps, say 36.2 pre-ov. and then post-ov. they go to 36.5; others start at 36.5 pre-ov. and go to 36.9 post-ov. IYSWIM. Usually post-ov it at least .2 higher than pre-ov but that's no fixed rule, and sometimes it's much higher.
What worrisome is talking about is a further temperature surge at about the time when AF would have been due, which is very common for pg women.
Thing is, as far as I understood: The remains of your follicular body, from which the egg emerged, produce this hormone which makes your temp go up - which is why temp usually rises shortly after ovulating. It keeps producing this hormone until the remains are all used up. If by this time the egg has reached the womb and has managed to implant, the womb takes over and produces this hormone all by itself, thus keeping the temp high and sometimes sending it even higher than it was already. If however the egg does not manage to implant, for example if it was not fertilised, then the hormone simply runs out, and temps go back to the lower level, usually around the time when AF arrives.
Last cycle, my temp also surged upwards, two days before AF was due - I tested immediately and sure enough, got a BFP! Unfortunatly I MCd a week later. This cycle, my temp is also going upwards, but HPT gave me BFN. So I'm expecting it to drop soon - if it stays high or goes even higher, I most certainly will test again!
Sorry for the monster-long post...