Take a look at this spreadsheet what I would like for a solution would be column F calculating this dynamically as I add new information to column a-e. so if you look at examples 1 & 2 you will see what the percentage should have been in columns a-f would have as of the dates in history that are in the examples.
Initially customer 11 was on line 1 2381ft he was at 100 percent of line use on 3/1/2010. then on 1/1/2012 customer 13 (example 1) came on to line 1 at the 500 foot mark, the two then share 500ft and customer 11 was responsible for 100 percent of the remaining line. Then on 10/1/2016 customer 14 joined the line at the 300 foot mark so customers 11,13 & 14 shared 300 feet customers 11&13 shared an additional 200 feet and customer one was solely responsible for the remaining footage. So again i would like this line percentage calculated in column f as i add more and more customers to the various lines book1.xlsx