Line share calculation

This seems very complicated to me and I am concerned I might just be overthinking it.  I have my results for this simple example in rows j:o
Book2.xlsx
But I really need to come up with a way to get these results as I add more Customers at various footage marks on the line.
-cost per foot of line is 5.95
-first 300 feet is free
-as customers come on to line the footage responsible gets shared with all up to that point  then shifts to the next level of customers responsible and that level is shared and so forth until there is just a remaining length shared solely by the last person on the line (you will see column g & h adjust as I add more customers to this line) this is the key getting a summary that really adjusts with these numbers as more customers are added to this sheet.

I am open to tweaking the set of the sheet whatever you think might help me make this work.