hello,
I was told that you need to take your temps at the same time every day, and take your temps before you get up to pee, or before you take a sip of water or do ANY activity as you are measuring your body temp AT REST.
That said, you can play around a little with the timing, eg: If you normally test at 8am, and you test one day at 8.30 you need to increase your temp by .5. So for every 30 mins increase your temps by .5, and visa versa if you need to wake earlier than 8. but the most important thing is to not do any activity at all before taking your temps...

good luck!