One idea is to tape a piece of card along the edge of your easle so it blocks the light from hitting the edge of the print. Cut notches where you'll be making the 'test strip lines' so there's a V of density at that point on the print...then you'll know where the changes are.

An even more anal way is to do that for both the top and bottom of the easle, just to make sure all the test strip lines are parallel...not that I've ever done that...nope...not me

Murray