I just finished setting up the strongback for my Guillemot. I found that the easiest way to make small adjustments was to fasten the form to the mounting blocks on the strongback with screws and mark the edges of the mounting blocks on the form. Then clamp the form to the blocks and remove the mounting screws. Now with guidelines on the form I can tap it with a hammer to nudge it a sixteenth left or right, using the pencil marks as a guide. When I've got it where I want it, I screw it down tight through new screw holes.

To follow up on something George said, I used lots of long thin wedges to move the forms around on my strongback until they were in alignment. It's pretty hard to hold everything in position by yourself and screw the form down. The wedges let you fiddle the form as much as you like before screwing it into place.