A simple pixel ratio only works reliably when the ruler and the object are in the same flat plane and the image has little distortion. What throws it off is mainly lens distortion and perspective.

To improve accuracy:

• Put the ruler directly adjacent to the feature you want to measure, on the same plane.
• Correct lens distortion first if possible; many editors can use lens metadata and lens profiles to reduce it.
• For better accuracy across a larger area, use more than one reference. Two rulers at right angles are better than one.
• Best of all, place references around the object—effectively making a grid. Then you can measure locally instead of assuming one pixel-to-inch ratio for the whole frame.

The key idea is that scale can vary across the image, so one ruler measured in one spot may not represent another spot perfectly. A grid or multiple rulers lets you account for that variation much more accurately.