I = TPD/2

TPD = Total Planned Distance
I = most likely place you will have an incident

Example:
Total planned distance = 58 miles.
Most likely place for an incidnet = Mile 29

At mile "I" you will get that emergency phone call that says, "We need you right away". Or your derailleur will gag and you have ride in one gear the rest of the way. Or you will have a minor crash that is just bad enough that you can still ride, but it isn't fun anymore.... Happens every time!