In 1957 a gallon of milk cost 0.97 cents. Today, it costs 3.20 $. What is the rate of inflation of a gallon of milk for the period?

So, if I understand it correctly, {via referencing google for the formula}. Then my calculations should go like this:

In 1957 the inflation rate was 100%, today the rate of inflation would be 100% times the price of milk, 3.20. Which would make, obviously, 320. So to continue finding out the rate of inflation, I would subtract the number 320 by 100, and get 220. Now, I divide that number by 100 to get the percentage, equaling 2.2, or 220%.

Did I do this right, or am I just confused and stupid?