How do I calculate percent change

In the phrase

"has increased from 12.1 percent to 20.4 percent"

I would like to state that "disease" has increased by a certain percent. The difference here is 8.3%. So I need to know how I word this AND calculate it.

If I divide 12.1% by 20.4% I have a change (increase) of 59.3%

If I divide 8.3% by 12.1% my change is 68.6%.

Which is the most correct way to present the increase?

Which is the best wording to express that change:
"disease" has increased by xx.x%
or
"disease" has increased xx.x%

Thanks