Tub surrounds rarely have perfectly square walls. Even if the walls are level, the end walls often bow in toward the tub bowl. And sometimes the tub rim isn't level, but tilted at a slight angle. The technique shown below will help you deal with this inevitable imperfection. It lets you make a reliable map of your goofy walls that you can use to cut and shape the toughest panel (the back one) so it will fit as if it were made for it.

DRAW a level line (using a 4-ft. level) indicating the top of the new panels. In the center of the back wall, draw a plumb line and check it for squareness with the level line using a 2-ft. framing square. On both these lines, make hash marks every 6 in. Measure from each hash mark to each corner and the tub top (both sides of the center line and down to the tub) and subtract 1/8 in. for the correct gap and dimension for the panel.