I've been weatherizing my house frantically. The salvaged French doors needed some repair along their bottom edges. Here's the tools I used. The wooden piece is nailed in place and provides a guide for when I cut the door. Square is difficult with salvaged doors which have been shaved. I looked to the hinge side to be straight and measured down off the muntins / molding to get an even cut across the bottom. The framing square checks for sanity. Measure twice cut twice, I always say.
In the category of everything old is new again, this is my favorite "new" tool which I received at a garage sale for free. It's an old folding rule. I thought for years that you'd use this tool just like a tape measure, and ignored it as too old and clunky. Little did I know that you unleash the real power of this tool when you use it as a story pole. Simply extend the brass slide out to fit inside dimensions and take it over to your work to mark the correct size right on it. This is FAR more accurate than transferring measurements made with a tape.