PhotoRule
Measure in a photo. Give it one length you know, or a rectangle you know to correct for the angle, then click across anything on that surface.
2 Measurements
Set the scale, then click two points on the photo.
One length
Fine when the camera looks straight at a flat surface. Put the known length on the same surface as what you measure, and as long as you can.
A rectangle
For a surface shot at an angle. Click its corners in order around the edge: the first edge is the width. Everything measured on that surface is corrected. Anything off it is not.
Precision
The loupe shows the pixels under the pointer; drag any point to adjust it. Lens distortion is not corrected. For calibrated work on a live camera, see Pixosight.