Question: how to do a running percentage in excel?
Answer: In cell C2, enter the formula =B2/$B$18. ... 
Copy C2 to just the next cell, either by dragging the fill handle down one cell or using Ctrl+C and then Ctrl+V.
Press F2 to edit cell C3.
Type a plus sign and touch cell C2. ... 
Double-click the fill handle in C3 to copy this formula down to all the other cells.