Forcing an Assignment Notify if Incident is rated Urgent - Otherwise leaving it to the Analyst to decide

A while back I was asked to standardise the format of our Assignment notifications across modules and the contents of this article served me really well. How to standardize Assignment notifications using calculations

Inevitably the request for a couple of tweaks have come.

I've been asked to force an Assignment Notification to the Assignee if, but only if, the Incident is Urgent. For all other occasions the Analyst must be free to Notify the Assignee or not. To decide whether the Incident was urgent I was going to use our Urgency Justification attribute which only appears when an incident is set to urgent. (Just in case the description of the Urgent Priority is changed in the future)

We're using the Process Assignment object for calculations.

I'm happy I can identify the Incident Class with a:

if ProcessAssignment.Process.Class.Name == "Incident":

I'm OK with embedding details from the Incident into the Email.

Where I'm getting stuck is with forcing the true value into the NotifyAssignee.

So in pseudo code what I'm trying to do is:

if ProcessAssignment.Process.Class.Name == "Incident":

       if Incident_UrgencyJustification != null:

               Set the NotifyAssignee boolean to be True and ReadOnly                            (To prevent it from being un-ticked)

       Else

               Let the Analyst decide whether to Tick the NotifyAssignee or leave it off

Hopefully I'm missing something really obvious, but any advice would be most welcome.

Thanks.