Divisiones con varios Checkbox... no sale :(

[America|UNK]

Código HTML:

<form name="calc">
Bj <input type="radio" name="box" value="Bj" checked />
Mer <input type="radio" name="box" value="Mer" />
Sod <input type="radio" name="box" value="Sod" />
Inc <input type="radio" name="box" value="Inc" />
 
<br /> <input type="text" name="watt" />w
<br /> Res: <input type="text" name="res" value="0" />
</form>
 

Código Javascript:

var vals = {Bj:2.5,Mer:3.2,Sod:2.5,Inc:8}
var form = document.calc
 
function calc(){
    var i = 0, obj
    while(obj = form.box[i++]) if(obj.checked)  form.res.value = +form.watt.value / vals[obj.value]
}
 
var i=0, obj;
while(obj = form.box[i++]) obj.onclick = calc //calcular al clickear boxs
 
form.watt.onkeyup = calc //calcular al escribir watt