I would start when you get it also even know I don't think it will tell you much until you have been charting for a whole cycle. At least if AF arrives when your charting this cycle (praying for you it doesn't arrive as your pregnant) you will have it to compare to your next cycle if need be just for those last few days leading to AF. My temp also's dips the morning AF arrives so as soon as I know my temp is down, surf board goes on (pad) and sure enough AF arrives within a few minutes/hour. Some peoples gradually drops over a few days whilst others dont drop until after AF has arrived. It will also tell you for next cycle what your temp is after ovulation so when you start tempting at the start of your cycle you should know what your temps have been in the luteal phase (O-AF)