This simple app only asks for your childs age then can be used by placing your phone on the child car seat and it will let you know if the angle of recline is appropriate or not. If the angle is not appropriate the app will suggest some changes to make.
The app includes no ads or annoyances and is available open source.
The source code and bug tracker for this app is available here: https://github.com/JohnMcLear/Child-Car-Seat-Angle-Tester
My research on this topic can be found here: http://mclear.co.uk/2016/08/25/car-seat-angle-tester/
Thanks for trying it out and I hope you find it useful to keep your children safe.