@ibjunkman:
I have never tried it on 78. Your iPhone would probably stay unharmed and on the platter if you laid it close to the center of the platter.
I really don´t understand your objection- would you rather not know if your turntable speed is way off?
The RPM app is just a simple and reliable way of knowing if your turntable operates at the right speed.
What could possibly be wrong with that? IMO it´s a basic parameter when playing vinyl.