Certainly, running at 33.333333333333 . . . with a stylus in the groove is ideal. Running an iPhone app I just discovered (RPM), I'm running about 33.55 or 33.56. Table is an LP12 with an 18 year old Lingo. Everything sounds OK- is it ok?
By most accounts, the iPhone method is not the most accurate way to measure platter speed, but it puts you in the ballpark. So I would not conclude that the speed error of your turntable amounts to the difference between what you measured and 33.33 RPM. It might be more, or it might be less.
(warning: reddit formatting for forum readability!)
I'll share my experience with the iPhone app (RPM, and Turntabulator) on my iPhone X. I decided to setup a Clearaudio Concept I'm evaluating. The Clearaudio provides the ability to use set screws to tweak the speed.
So, I set the iPhone X top touching the platter pin, and gave it a spin. I then tweaked until I was nailing 33.33 RPM.
I then decided to lay the iPhone "sideways" on the platter, and RPM showed a different speed in each place. Sometimes off by quite a bit (~35.4 RPM)
So, I went back to the first way, and decided to play a reference song of mine ("Nutshell" by Alice in Chains from Unplugged). I know that song cold, and have heard it on systems literally costing $500k plus both digital and analog.
I was dismayed to hear the Laine's voice sounded high pitched, and everything sounded off. How was this possible when RPM said the turntable was running pretty damned nice at 33.33 RPM?
Frustrated, I then took out my trusty "The Ultimate Analogue Test LP", and played a 1kHz tone from side 1, track 1. I used "n-Track Tuner" which is an app made for guitar / instrument tuning... 1056Hz?! Jesus!
So I decided to tune the 33 1/3 RPM speed of the Concept down until it showed about 1000Hz for the 1000Hz tone. Once done, I re-played my reference Alice in Chains track... perfect. :)
Pitch stability is MUCH MORE about off-center holes than variations in properly performing turntable motor operations. The difference between 33.31 and 33.35 is insignificant as long as the speed remains relatively constant in the short term. Unless a listener is absolutely pitch perfect (very tiny percentage of listeners), they won't know the difference in the long term. Even cutting lathes didn't maintain perfectly stable speed rates. (The Beatles "Golden slumber / Carry That Weight / The End" seriously changes pitch increasing from slightly flat to quite sharp during the recording, but it quite listenable.)
In my opinion, the REAL issue is not about speed accuracy for A/C driven turntable motors. It is ALL about A/C noise making its way into the platter or plinth. There is a quite noticeable difference in noise entering "the system" when the turntable drive motor A/C voltage (VPI Aries and VPI modified TNT) is reduced by a variac from 120v to around 75v. That is still enough voltage to maintain stable platter rotation to operate the table but the vibration generated by the A/C synchronous is dramatically reduced. This can further be demonstrated by using different kinds of drive belts. Reducing this noise floor makes a big difference in detail, soundstage, image tightness, instrument character and performance subtleties.
Just my opinion and personal observation (as a former professional performing musician). Your opinion may differ, your opinion may differ.
I think you and I have gone back and forth on this subject in the past. The issue with the voltage going to the turntable motor could be of two sorts. In one case the motor can directly radiate EMI which can affect the cartridge. This would mostly be a problem with direct drive turntables where the motor is installed right under the platter. In the second case, the AC interference from the motor goes back on the AC line and goes to your other components via The AC line. This latter problem can easily be avoided by isolating the AC supply to the turntable from the AC supply to your other components, particularly your phono stage, line stage, and amplifier. Reducing the AC voltage supply to the turntable by as much as you suggest, going from 120 V to 75 V, can have a deLeterious effect on the performance of some motors. Because at some point the torque will drop dramatically.
I also kind of disagree with the idea that the speed accuracy of the turntable needs only to be as good as that of the cutting lathe. Because if the turntable speed constancy is poor, then you are adding an additional source of error to the speed at which the record runs. This error in the turntable could be additive with the error of the cutting lathe, in which case you are losing some fidelity that would otherwise be obtainable.