This is very nice. I'm experiencing some problems taking the picture directly by the camera (Nexus 5; 6.0.1). It takes a picture but the picture is empty or broken (but still shows up in the gallery). As a workaround, it's possible to make pictures by the default Camera app and then select the picture from the gallery.

Anyway, I think it's quite useful and surprisingly precise (+/- 1%). If I may make a feature request, I'd like to see more reference sheet sizes, e.g. the full A0-6 range.. :)