ciao beppenet,

tipo c1=1000 e c2=5 deve risultare 950?
codice:
<script language="javascript">
function NomeFunzione(ff){
  var euri = parseFloat(ff.C1.value)
  var perc = parseFloat(ff.C2.value)
  ff.C3.value=euri-(euri/100*perc)
}
</script>
<form>
Euri<input type="text" name="C1">
- %<input type="text" name="C2">
<input type="button" value="=" onclick="NomeFunzione(this.form)">
<input type="text" name="C3">
</form>