Story sticks are the savior for guys like me who have a measuring disorder. No matter how precisely I use a tape measure there always seems to be some drift, I am often off by a sixteenth or so. A story stick allows me to measure perfectly the first time.
The crazy part is that it is so darn simple.
All you need is a few pieces of thin scrap. Molding works, maybe some pieces you have left over rafter ripping on your table saw. Take two pieces of this scrap and cut to roughly two thirds of the distance you want to measure. If you want super precision you can cut one end of each piece at a 45 so the come to a point. The only other thing you need is a small clamp, one of those binder clip things or if you are in a pinch tape will work.
All you do from here is arrange the sticks so that the points are on the same side pointing away from each other. place the in the space you want to measure and slide them so that one point from each stick touches each side of what you are measuring. use you clamp, clip or tape to secure the sticks right where they are. carry the stick to your board, mark the length and cut.
Works like a dream and only costs about 30 cents.