Measuring TT Speed

its a bit extreme but the proper way to measure is with a calibrated test tone disk and a frequency counter. You can also send the output to a scope and visually watch the deviations.

jsman,
yes, it's the illuminated number icons that count. If there's drift, and you have a way to adjust platter speed, you can usually get the illuminated numbers to hold steady, plus or minus. I always make speed adjustments with the tonearm lowered and playing the first song of a LP sitting under the strobe disc. It's surprising how much stylus drag there is.
If you don't have a way to adjust speed you can get a close estimate of actual speed and error using this technique posted at the KAB website:

TO EVALUATE DRIFTING SPEED, [COUNT] THE NUMBER OF ICONS THAT MOVE IN AND OUT OF THE SPOTLIGHT IN 60 SECONDS. 

ACTUAL SPEED = 1 + (#ICONS DRIFT PER MIN / 3600) X RPM 

DRIFT IN PERCENT = (#ICONS DRIFT PER MIN / 3600) X 100

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.

@johnss:  Correct.  And that is what you get with AnalogMagik.  But still there are potential variables, including proper center hole on test LP, warp, etc.

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

@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.