Manual Entry Not Visible in Overwritten Field

So, I've used a script posted by Max Wyss that allows me to overwrite a field that contains a calculation (using a control checkbox that removes read-only when it is checked). The script works, however after I check the control box, what ever value I type into the field goes invisible once I hit return. The value is there because it shows up in the grand total at the bottom of the page. Is there a trick to making the overwriting entry visible? I'm pasting Max's original post here:  

Let's assume that the field to be made overwriteable is called "myResult", and the controlling checkbox is called "myResultCtrl".

The checkbox will get the following script in the mouseUp event:

if (event.target.value == "Off") {

this.getField("myResult").readonly = true ;

} else {

this.getField("myResult").readonly = false ;

}

In the calculation involving the myResult field, you will add the following structure (you may have to adjust it for the calculation)

if (this.getField("myResultCtrl").value == "Off") {

// regular calculation code for field myResult comes here

} 

And that should get you closer to the intended result.

Hope this can help.

By Max Wyss