Those input percentages appear to be exactly linear, so I would probably derive the equation for that straight line and use that. This might be easiest (since you probably don't actually need to know the equation) using the TREND() function. http://office.microsoft.com/en-us/ex...832.aspx?CTT=1
1) Put the desired "fixed" values in a table. The TREND() function does not like blanks, so you will need to set it up accordingly:
2) To calculate the percentage at some "new_x", enter new_x into a cell.
3) To calculate the percentage at that x, enter =TREND(known_y,known_x,new_x) into an adjacent cell. known_y etc are references to the appropriate cell(s) in the spreadsheet.
Note that this only works as long as the "fixed" points form a perfectly straight line. If it doesn't form a straight line, then you will need to further define how you want to calculate the percentages.