You need to know when you ovulated before you can reliably determine when to take a pregnancy test. Your BBT chart can help you determine how many days past ovulation you are. You can take an HPT when you are past your usual luteal phase length (the time between ovulation and when you get your period) but if your usual luteal phase length is shorter than 13 days, then you should probably consider waiting a day or two longer.
Once you have a full cycle charted in Fertility Friend, a reasonable test date will be proposed to you based on your cycle statistics. If you do not have a full cycle charted, the software will recommend waiting until you are 18 days past ovulation to avoid the disappointment of early negative or false negative results and wasting tests.
Whether or not you test earlier than is recommended really depends on your attitude towards seeing negative or false-negative test results. In general, results are usually accurate once you are 14 days or more past ovulation but if you want to avoid wasting tests and your luteal phase length is longer than that, or you do not know your usual luteal phase length, waiting a bit longer is recommended.
For more information about pregnancy test timing, please see our Pregnancy Test Study.