my idea,not sure if every one would approve.Trace the outline of the stab on a piece of paper ,or use the outline thats on the plans --Fold it in half -(end-to-end) .The crease is now the center line of the stab. With this pattern -trace the centerline on to the built stab.Find center of fuselage --align marks --With stab pinned to fuselage-sight down fuselage to see if its centered up & down.If the bottom of the fuselage is flat --just measure both sides at the ends to the table ----