I'm sure I know how to do this but its not working

For every week people get performance rated, ie 1 for good, 0 for bad. I
have a table for this information.

What I want to do it use the following table to apply performance % for qtr.

Score
13 100%
12 80%
11 60%
10 40%
9 20%
8 0%
7 0% and so on.

I have tried using a sum to add the ones, and then a VLOOKUP, but it doesn't
work!! HELP!