Using vlookup to return single value based on calculations of other columns?

Hi, I have attached a spreadsheet where I am attempting to return a single value (highlighted in yellow) for a calculation. i cannot assume the data each month will always fall on the same row so i need to use lookup to ensure i am adding the right data. Does anyone know how I can do this?

thanks

If you change B20 to read just S and B21 to read just M, then in C20:

=SUMIF($B$3:$B$17,B20&"*",$I$3:$I$17)/SUMIF($B$3:$B$17,B20&"*",$E$3:$E$17)

copied down.