Measuring TT Speed

@edwyun .... re. “Some RPM apps are better than others. The preferred ones have a way to be calibrated“

Please name some of the apps that have a way to be calibrated.

+1 on the Timeline

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. 

Your way is fine, and so is the KAB strobe.  It is quite a precise instrument in use, and a speed error is immediately detectable.  This is really not worth further discussion, in my opinion.  I learned something from your last post; maybe failure to center the cell phone is a reason why others have reported problems with such apps, and maybe the RPM app is superior to some other cell phone apps, as well.  I am OK with that.

@ml8764ag
The RPM Turntable Speed Accuracy/World TT Database app for iPhone has a way to calibrate the app and then applies a multiplier for better precision.

The RPM Speed and Wow app and the RPM Calculator app both for android, which are popular, dont have any way to calibrate the app, last time I checked.