When working short handed I have had some success using small sticks clamped in place perpendicular to the stringer to lock it in place during a dry fit in addition to the pencil marks. That way when you get the part all smeared up with goop and you can't see any marks, and you are in a hurry, you have some confidence that the stringer is close to the marks. Then when you have time you can smear away the goop, find the marks, and remove the temporary sticks.