Measuring TT Speed

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.

Mijo, You are certainly OK to prefer and use the RPM app on your cell phone, but your little experiment doesn't prove anything about its accuracy relative to an un-named strobe to which you compared it.  I am only speaking for the KAB strobe, not strobes in general. I got the idea not to trust cell phone apps based on numerous posts to this forum and the Vinyl Asylum website where users complain about their lack of accuracy, or they had a problem with tt speed that ultimately was proven to be related to their reliance on a cell phone app.  Like I said, I don't recall whether anyone specifically named the RPM app.

Mijostyn, With much respect, especially as I have never used the particular application you recommend, RPM, cell phones are notoriously inaccurate for measuring TT speed.  If the strobe is wavering back and forth, it means something. Sometimes that can be an optical trick due to flickering of the strobe light or see below. So long as the strobe markings do not move definitively in one direction or the other, the problem may lie with the strobe.  The OP does not say what strobe he or she is using.  For me, the only strobe worth having is the KAB product, which comes with a battery-powered strobe light that is constant in its pulse and a larger than average diameter strobe, so the markings are moving at a faster speed due to a further distance from the center of rotation, compared to the average strobe disc.  Any strobe light that plugs into the wall is subject to the minute variations of the AC frequency and could give the phenomenon the OP is talking about. Unfortunately, the KAB kit with strobe disc and battery-powered strobe light costs $100, but is well worth it in my opinion.