Rise time is normally measured as the time to go from the 10% amplitude threshold to 90% amplitude threshold of the waveform.
For your example:
Amplitude = 86 - 20 = 66
10% threshold = 20 + 6.6 = 26.6
90% threshold = 86 - 6.6 = 79.4
You are going to have to do some interpolation to determine the times wen the signal reaches 26.6 and 79.4 (or take the measurements more frequently).