Best way to update selectedIndex variables?

Best way to update selectedIndex variables?

http://jsbin.com/obuko4/edit

This is my sample code so far. Currently it only works for "Akali" under the "Select a champion..." dropdown list.
The way it works is, you select a level from 1-10 from the "Select a level..." dropdown list, and then you select Akali from the other dropdown list and her "Health" is calculated by "lvl * 80 + 510" which works perfectly. But the problem is if you change the level that value does not update automaticly. Instead the person using it has change the level to the new level they desire, change the champion and change it back again for it to update. Which normally would be ok, but I will be adding much, much more data to the page and I can't ask people to have to update stuff 10 times just to get accurate figures.

I took this from the w3schools jQuery tutorial and built a button, so ignore the way it currently works.
I guessed that with a button it would when pressed it would take the latest, aka current selectedIndex values and all would work fine, and it does actually. But I just wanted to ask if there was any way to keep all the information updated at all times without a button, or maybe a button that will update everything on the website with just one click? Thank you for reading this and thanks in advance for any answer.
  1. <h2>This is a heading</h2>
    <p>This is a paragraph.</p>
    <p>This is another paragraph.</p>
    <button>Click me</button>

    <script type="text/javascript">
    $(document).ready(function(){
    $("button").click(function(){
    $("p").append((60 + (idmg * 0.75)) - (60 + (idmg * 0.75)) * (eiarmor / (eiarmor + 100)));
    });
    });
    </script>