It's not clear from OP's initial post whether the +/- 15% is the accuracy or the precision. Actually I suspect it is the accuracy. I imagine it doesn't move around a lot during use, but the approximations evident in entering your weight and "type" of bike as well as your personal drag coefficient could easily cause a relatively FIXED 15% error. Yes that would be a problem the first time you used it. But if you could possibly calibrate your output under standard conditions to a power meter as compared to the app (more or less), you could transfer that calibration to the app and determine the fixed error you are dealing with. Then just make the necessary correction to the results.