validations.js

document.observe("dom:loaded", function() {
  $$(".things_you_want_to_validate").each(function(item) { 
    if(item.value) {
      var x = parseFloat(item.value);
      var handler = function(event) {
        var element = Event.element(event);
        var old_val = x;
        var new_val = parseFloat(element.value);
        if (isNaN(new_val) || new_val > (old_val * 1.2) || new_val < (old_val * .8)) {
          element.addClassName('warning');
        } else {
          element.removeClassName('warning');
        }
      };
      item.observe('blur', handler);
    }
  });
});