A long cut with the circular saw, you want a backstop. How to place the backstop perfectly? Well here is the tool..
Step 1: All You Need Is a Paintstick and a Pencil.
mark on a piece of painstick from the end where the blade is gonna go. drill a hole for a piece of string to attach the guide to the saw, and now you can measure easily from the backstop piece of wood to the cut line. Its that simple.
Step 2: If Only the Guy That Built My House Would of Had This Tool...
I wouldn't have to deal with walls like this in my house. ;)