Measuring TT Speed

In measuring my tt speed I am seeing the lines on the strobe bounce back and forth, not holding steady. This is a belt tt with a dc motor setup with a closed loop controller, and ideas on what can cause this? I am thinking the belt may be to tight, it is a new belt BTW...
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.
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.

Have you ever been aware of a speed change when listening? If not, try to ignore it.

Wonderful Thorens TD 124, idler wheel drive, mirror constantly reflecting strobe on underside of platter: each session the motor had to warm up before you adjusted it to get the built in strobe lines steady. then after a while, check, might be a speck slow or a speck fast.

Not that I heard it, just when changing lp's I saw it. And that's a 7lb or 9lb platter to behave like a flywheel, with the most precise bearing I have ever seen.

Direct Drive, not quartz, similar, warm up needed, but steady after that.

Direct Drive, Quartz Locked, ON, walk away.

The speed controls are nice, but wait 10 minutes for warm up, and then only a minor adjustment should be needed, their larger range is for purposeful audible changes.
I have yet to see actual W&F measurements for a DD table that is any better than most other tables. Published W&F measurements from the manufacturer, sure.