The app is very nice and useful. UI is user friendly. This apps lets you measure the correct dimensions of the things which can be placed over the phone screen. Things which can't be placed over the phon screen can't be measured. To overcome this following can come handy: Use camera (not photo or picture) for that. While camera will be showing the things, it can be marked and measured. (Although accuracy will not be 100%)
When you open it , it may looks inaccurate. But you can adjust it to fit your phone. Go and click calibration and bring your real ruler and put it on the phone and compare and adjust it. Push or pull your screen with finger and save. After then, it is very useful app to me. I love this.