fertilityfriend.com is a great help, I agree. Most women do see a drop in temp a day or 2 b4 af starts. There are some women, myself included, who's temps don't drop until a day or 2 after af shows. Once you know how long your Lp(leutal phase)is, it will be easy to see the pattern as to when af will(or hopefully won't) show up. Remember to always take your temp at the same time every day b4 immediately upon waking and b4 getting up.
Try going to fertility friend. It is such a good help. Your temperature would usually be low and the day before it goes flying high is when you had ovulated already. You have to look out for the day b/4 the high one starts. From there it stays high until you get your period or it stays high which means you are pregnant. It's not that hard but you do need help to recognize your chart and pattern. Hope that helps.
Temperature increases at ovulation (or is it a day before???). Get a chart from the internet or your obgyn/RE. Once you know when your periods occur and how long your periods last, you'll know your cycle day count. For example, my period starts every 29 day (maybe a 30-day cycle from time to time). After I got into the pattern of knowing my cycles, I decided to start seeing when I ovulate. What I'm saying is, BBT is useful for finding out when you O, not when you'll start your period. But that's just my opinion... it's like your trying to do the BBT thing backwards by the way you stated your quesiton.