You can use just about any method, just as long as you are consistent on how you measure. That way you can see if you stats have increased, stayed the same, or God forbid….decreased. (Yikes!)
If you really want to be accurate so as to compare to people on the forum, then follow what Slack said, this is the generally accepted method for measuring BPFSL.
The whole point to finding your BPFSL is the “potential” growth factor. A lot of people, including myself believe BPFSL is an indication to future gains. BPFSL is also a lot easier to get a consistent measurement where as PBEL will vary greatly by how aroused you are when you measure. So far, I have found that when I see my BPFSL gain, my BPEL will catch up with some edging and jelqing exercises.