Finish trim work around windows or doors requires accurate inside measurements, but it’s hard to get precise readings with a tape measure. The “see through” windows on some tapes don’t work very well, and I’ve never gotten accurate results from measurements that include the length of the tape case.
Instead, I use both my tape measure and my combination square, as shown. The square’s blade butts against the inside surface, while the tape measure records the distance from the opposite edge to the blade. I add 12 in. to the tape reading for the total. This method is fast, precise and makes use of the tools I’ve already got in my belt.
Philip Zimmerman, Berkeley, CA