SETTING MEASUREMENT PARAMETERS
When using the Analysis tools in Photoshop, you will need to establish the scale of measure. Go under the Analysis menu, choose Set Measurement Scale, and select Custom. You have three settings to modify: Pixel Length, Logical Length, and Logical Units. An example would be if you know your document is 100 pixels wide, you would enter 100 in the Pixel Length field. Then, if you know that 100 pixels equal an inch, you would enter 1 for Logical Length. Then type inches for Logical Units. Now you have set the Measurement Scale to measure every 100 pixels as one inch.