I've started cutting dadoes by hand for the first time to fit shelves for a small pine bookcase. To my eye, they're far from perfect (occasional deviations from squareness in the walls and small visible gaps in the joint fit on the ends, for example), but I can fit the shelves by hand (or with some light mallet taps), and the fit is tight enough that I can pick up the shelf and the joint stays intact. I'm going to go out on a limb and guess that this is tight enough to glue well, but is there a good rule of thumb for testing the fit of a dado? I'm planning on building some bookcases out of less forgiving wood than pine, and I'd rather not have them self-destruct when full of books.