I believe the iPhone App is accurate, but the length is short compared to a two foot level but closer to a torpedo level. Experiment with where you place it to check for level.
When manually leveling, the light does not go solid but flashes when level.
Manual leveling can aid in eliminating raising a wheel off the ground. I level the low side and then run the opposite side down to where the jacks just hit the ground. Then level the low front or rear end, the opposite end is already touching the ground so there is no need to change that end. As many have noted in other posts, the final definitive test is whether or not the bathroom door stays open or closed.