The modern tape measure is a pretty remarkable tool. Some of the wider ones, like the one used here, have a standout (the length at which they can extend and stay level, without collapsing) of almost 10 feet. That can make taking measurements much easier when you're working by yourself. But tape measures seem to have their limitations.
A big limitation of these tape measures becomes evident when measuring the distance between two inside angles. Usually, the best you can do is roll them into the corner and use a bit of guesswork.
Here's a simple way to use a tape measure and get an accurate inside measurement.
First, grab a framing square, sometimes called a carpenter's square. These have two sides; usually one is 16 inches and one is 24 inches. Lay the framing square flush against one of the inside angles.
Use the tape measure to measure the distance from the end of the framing square to the other inside angle. Then simply add the length of the square (say, 16 or 24 inches) to this measurement.
By combining two trusty measuring tools, you've increased the usefulness of both!