When I try to create Visio shapes from a sample image, I import the image into Visio and stretch it so it matches the scale of the drawing. I can then use the dimension shapes to "measure" the image.
Recently I came across a collection of shapes that had similar profiles, but the proportions varied. I needed a new kind of measuring tool, one that would display percentages. With Visio, this is relatively easy.
The shape is a simple line with a control handle with a user defined field set to "=GUARD(Controls.Point/Width)". The text field for the line contains the user defined field. The line is strectched across the distance that needs to be proportioned and the control handle is moved to various positions. The text of the line will show the percentage of the distance that the control handle is at.
I had considered constraining the control handle to the line, but the freedom of movement makes it very easy to "measure" irregular shapes.
John... Visio MVP