When should I take my temperature? I know it is midcycle but...

When should I take my temperature? I know it is midcycle, I am in my mid-forties , and (although in our family have been told the menopause is around 54 years) my periods are very scanty, it’s difficult to tell when they begin and end e.g last time I started slightly on Sunday ,I think, and finished arond the following Monday (9days) in the middle I have a couple of days (two or three) of light bleeding. Do I count two weeks from the middle of those days? They are still fairly regular, about 26 to 28 days cycle.