Morning fella.
When I did my test my instructor had a bay like the one for test conditions and what we did was made a loop with one of the straps on the trailer and as I looked in the mirror I just had to make sure that the loop was lined up with a certain point (think it was my shoulder in the mirror and that was lined up with a bollard) then that meant that the rear of the trailer was about 1or 2 inch from hitting the stop sign at the back on my test, worked a treat and passed first time 
Good luck you’ll be fine and like others have said don’t be over thinking it