Item visibility based on addon property

Sorry - probably a newbie question, but I've trawled online for hours to no avail!

I would like to set the visibility of an item based on one of the addon's configurable settings.

i.e. the element starts like this:

            <control type="group" id="500">

                <visible>Integer.IsGreater(Container(400).NumItems,0) + String.IsEmpty(Window.Property(drawing))</visible>

I have a boolean property for the addon called "ondeck".

Can I amend the above visibility based on this property?

Any help appreciated.