Lewm, You have to place the phone directly over the spindle for it to measure accurately. Most just lean it against the spindle so the phone is anything but horizontal. I use a record weight with a flat top so that the phone is right over the spindle and horizontal. If it is not dead center the wow and flutter readings will be off. Readings are repeatable and ease of use relative to a strobe is superior. How long do you want to stare at a strobe to see if it is drifting. You also do not get wow and flutter readings off a strobe. I use an iPhone by the way in spite of hating Apple. I have used several strobes. Why don't you give this a spin and see for yourself. If you can't get it to read accurately and repeatedly stick with your strobe. No big loss.
The phone does not have to be calibrated. It already has an accurate built in clock and this is a measurement against time, Revolutions Per Minute. Counting against the clock is an extremely simple function.
