Trying to write javascript code to a check box that when it is checked, it removes the readonly attribute from another field and allows a number to be entered vs. the default calculation occurring.

You won't be able to use the "simple field notation" method anymore if you need to make the field editable in certain situations. The following script assumes that you just want to add up two fields and use 25% of that sum as your markup, unless the checkbox named "CheckBox" is checked, then the field will become editable, and you can enter anything you want:

// get the state of the "CheckBox" - modify the field name to match your form
var cb = this.getField("CheckBox");
if (cb.value == "Off") {
    // Checkbox is unchecked, so perform the calculation and make sure the field is read-only
    event.target.readonly = true;
     // simple calculation - you need to modify this
    var result = (Number(this.getField("Field1").value) + Number(this.getField("Field2").value)) * 0.25;
    event.value = result;
}
else {
    // Checkbox is checked, so do not modify the value, set the field to read/write and let the user enter anything they want.
    event.target.readonly = false;
}