calculating claims expense

Hello. I'm calculating claims on 12 months of sales. Each month is different and the claims expense is 50% but it rolls out throughout the year. So, if I sell $120 in January, that month will have $5 of claims and so will the 11 months following it. This would be easy to do but since there are other months the claims expense ends up being a total. How would I calculate this using excel and formulas? I've attached an excel sheet. Thanks.