Or you can simply remember that percent (per 100, 100 centimeters in a meter, one hundred cents in a dollar) is a relatively simple math concept. Ten percent, for example is 10 per 100, or 10/100, or .10.
If one cares not to understand the math, simply move the decimal point two places to the left so that 6.5% becomes .065 and multiply this and your base number to get your answer.
For example 6.5% sales tax on $49.50 is .065 x $49.5 = $3.22.
I have no use for a percent key. I find it easier to remember the math concept than how to use a dedicated percent key.