I use the Mac screenshot utility a lot, but have never thought of using it as a ruler. I’ve always used a browser extension to measure pixels on the screen. If you’re not using this utility, you initiate it by pressing command + shift + 4 on your keyboard.

I learned a few other tricks from the thread. Once you start dragging your mouse to create the rectangle:

• Press shift to lock in an axis as you continue sizing the area.
• Press space to move the rectangle to a new location.

Do you have any other tips?