I've developed a little online tool that is intended to help calculate (and hopefully to help learn to calculate) percentages. I'm hoping to get some feedback from math students, teachers and enthusiasts. What do you think about it? What would you add or change? Or maybe there are some language errors? (I'm from a non-English-speaking country). The URL of the tool is: Percentage Calculator

The idea is simple: in a "x% of y is z" user fills in and two of the three fields and the third is calculated for him.
There are a few more forms representing exactly the same sentence, but the order of variables is changed. User can choose to edit any form - values from filled and computed fields are copied to all other forms. May seem confusing, but when you look at the site, it's pretty much self-explanatory.

Few notes:
* For the sake of sanity, I decided to round results to the second decimal place.
* When user inputs a 0 in a place where it would result in a division by 0, I don't make the calculation and display a warning (and a suggestion that they may keep on typing to get a number like 0.25)
* There are still some things to do (for example users still are allowed to type-in illegal characters, such as 'asdf')

I'm really interested in hearing your opinions... please use the site for a moment a tell me what you think