The accuracy of the level application on my one phone is out bt over
7 degrees. That is a simple "level" app.. The "rigid" level application on my Blackberry PlayBook is very accurate - How you reference it to the wheel will be the biggest variable that can through your accuracy off. A trammel type setup made from a straight bar of metal (or "straight" hardwood) with 2 screws protruding to reach the edge of the rim, adjusted to be identical in protrusion, will transfer the wheel angle accurately to the "level". You can determine if the rim is true to the spindle by checking the level with the bar upright with the wheel turned 180 degrees to make sure the reading is the same with the wheel turned.WITH CARE you can check your camber to a reasonably high level of accuracy. To get the camber "normalized" you need to roll the car back and forth a few feet so the car "settles" on it's suspension. A professional setup uses a "slip plate" that allows the wheels to slide in and out with little resistance. Normal procedure is to bounce the car on the slip plates to "normalize" the suspension.
I've done it long pre-smart-phone using a simple bubble level to verify the alignment was "close enough" afterr an accident in central Africa severely damaged the front of my Peugeot. I've also done hundreds of alignments with "pro" equipment.