With so many variables, I wonder whether you really can arrive at a rule-of-thumb answer that is safe to use across the board?
At the risk of sounding un-educated, there is a rule -of-thumb:
You should be at rotation speed by the time you reach the half way point of the runway.( which is kind-of-the same as 85% and 40% but easier identifiable)
Better yet:
T/O distance ground roll + landing distance ground roll + 20% = runway length required. Using the values published in the POH.