I believe you use the same method as adjusting a Beretta 92 rear sights.
Shooters at the range would come to me, as NRA instructor, complaining that their new Beretta show low and to the left. We're talking in the realm of 4 to 6 inches out at 25 FEET. I'd normally take it, shoot a full magazine through it, getting them all in the black, with no difficulty, at the same distance.
I'd then proceed to tell the shooter that we need to use a hammer to make the adjustment, and ask him to place his index finger on the bench top......
I believe that would work for fretless bass. Times four of course.