Which formula to use to calculate ratings

Good day,

I'm looking for a formula to use. I need to calculate ratings on variances in percentages:

Variance vs expected Rating
0% 3
1-5% 2.5
6-10% 2
10-15% 1.5
15%+ 1

The expected percentage is 5. In other words if a % below 5 is obtained the rating is 3. Between 6% and 10% the rating should be 2.5.

Thanks

Lucille