FCPX, the viewer, and secondary display

I saw where you explained this very clearly Luis ... but I couldn't quite translate it to what I'm trying to do. I want my MacBook Pro as the left screen, FCPX app as my middle screen, and the Full Frame of the Viewer Window on my third (right screen). That way my MacBook is used for all the ancillary work, like mail, web, Affinity Photo et al. – over on the left out of the way.

But when I set it up like this, and click the Full Frame display over the Inspector Window, FCPX displays on my MacBook Pro screen ... not my 4K BenQ over on the right. Here is a snap of how I've got the displays set up now. See attached. Perhaps you could say, 'aww Ben, just do this'. Which I will do immediately. Of course.