Cadet Murphy, are you serious??
I normally precalculate a 10 mile final-fix for my destination (since ATC is more than likely to position you at 10 miles when giving radar vectors).
I do this via a spreadsheet on the iPad developed specifically for this PBD calculation, and then insert the waypoints in the FMS during preflight. I can always delete later or simply use as reference.
And how would you do it if you didn't inserted it for whatever reason?