This box joint jig by Ed Stiles is brilliant. I’ve seen some very clever jigs involving gears but the 16 thread per inch screw and the cam stop makes this very easy to use and is sufficiently flexible for my purposes. I can make any combination of fingers and holes in 1/16″ inch increments using this jig. There are also plans for this jig available here.
As a bonus, looking at the wall of tools being Ed is an almost religious experience, especially if you’re a little OCD.