A Smartphone or something else with a touchscreen works aswell, or even a computer. You just have to resize the picture until it fits the size of the dart. Then you can take measurements from the fins.
Great thread I now have the sizing on the fins correct, but I have a question about attaching the fins. If using Dark Side's aluminum grappling dart and aluminum fins that I made what is the best way to attach the fins? Some help from anybody that has already done this would be great.