Hello,
You can use measurement gating to specify the area you want the scope to measure. To use measurement gating, double-tap on the measurement and then select the Gating tab. There are a couple Gating types, I would suggest using Cursors or Time. With these gating options, you will need to use the Multipurpose knobs to move the cursors or start/end gate time. This will allow you to measure just the first rise and fall of the signal.
There is also a Time measurement called Data Rate that I believe will do what you are looking to do. This measurement gives the reciprocal of the Unit Interval.
