I think there are some limitations to the phone's light sensor that might prohibit an accurate measurement of light intensity or darkness.
I was thinking more like an app where you could plug in your transplant date, how often you want to change the rez, prediction for benchmarks like when you should be getting to flowering stage, and be able to log the daily variables (temp, ph, height, etc.) so that after the grow you can see patterns and get a more scientific analysis why somethings went right and some went wrong