Charting basal body temperature (BBT) but temperature keeps rising after thermometer beeps?

Hello, I am charting my basal body temperature (BBT) but my temperature keeps rising after thermometer beeps. This happens on both of my thermometers. The temperature does stop rising about 30 seconds after the beep. Should I use the reading at the time of the beep, or the time that the temperature stabilizes on the thermometer?