Shelter 501 MKii/RB 600/Gyro SE: how many spacers?

Without actually seeing your turntable setup, it's hard to advise you -- unless one of our forum members has the same setup as you do and can provide advice based on personal experience. However, what you want to achieve is for the tonearm to be level, and if that requires spacers to raise it, then use whatever thickness is necessary.

I have a Rega RB-900 tonearm mounted on my VPI HW-19 with Super Armboard, and I had to use the 2 mm spacers under the tonearm to raise it sufficiently so the arm was level.

There are two ways you can check for level when the arm is down and the stylus in contact with the record:
1. Use a small bubble level that will sit astride of the tonearm, back near the pivot (to shift the weight of the level to the rear). If you can find someone who owns a Dennesen Soundtractor, perhaps they will lend you the little bubble level that came with the unit -- it is made to ride on top of the tonearm.
2. Use a self-levelling laser, like the Black & Decker models advertised on TV. Adjust the tonearm until the laser's beam just grazes the top surface of the tonearm -- and add a spacer as necessary to get the arm level.

Some cartridges seem to work better with the tonearm just very slightly down near the pivot, and others seem to "lock in" with the angle just slightly down at the cartridge end. FWIW, however, my RB-900 seems to work best when the arm is dead level.

However, what you want to achieve is for the tonearm to be level, and if that requires spacers to raise it, then use whatever thickness is necessary.

I must respectfully disagree. Whether one's tonearm is level or otherwise is almost completely irrelevant.

In this context what matters are the angles between the stylus, cantilever and groove. That's where vinyl music reproduction begins, not up in the air in the arm tube.

The easiest and most reliable way to rough in arm height is to find out out how other people play your cartridge. The angle a cartridge prefers will be the same regardless of what arm it's mounted on.

In the case of a Shelter, the most commonly preferred attitude is SLIGHTLY tail-down. Use whatever spacers you need to achieve that. (Make sure your VTF is set properly. Changing VTF alters VTA and vice-versa.)

Fine-tuning by ear from there is desirable, though with a Rega spacers-and-locking nut setup it's probably not worth the trouble. You'd need one of the aftermarket arm height adjusters to do any real tweaking.

Fine, Doug, disagreement is always legitimate. However, unless you begin with a level tonearm, you have no point of reference with regard to stylus "rake" angle. I have been setting up tonearm systems, both professionally and for friends, for nearly 40 years, and learned how to do this from one of the best analog experts in the industry. And at least two of the best tonearm protractors state in the instructions that it's best to start with a level tonearm, and then to adjust rake angle by ear.

As a further point in support of levelling the tonearm, I was in one of our leading high-end audio stores in Seattle (Experience Audio) about two weeks, and the owner (Tim Ratcliffe) was setting up a Shelter 901 on a customer's VPI Super Scoutmaster with JMW-9 Signature arm. One of the first things that Tim did (and asked me to double-check for him) was to level the tonearm -- and Tim is arguably one of most knowledgeable guys I know when it comes to analog front ends.

Tonearm geometry has generally been based on a stylus rake angle of about 17 degrees, which is referenced to the top surface of the cartridge housing, which must be horizontally level to be accurate.

So, while I recognize and respect your right to disagree, I must reclama that the most accurate way to set tonearm / stylus rake angle is by starting with a level tonearm.

I think the issue here is that it is, as DougDeacon states, more accurate to have the appropriate angles to the groove at the correct angles to the record surface. In practice this may or may not be when the tonearm is level. While many, including myself, may start with the arm level this position is based on an assumption that the 'table, arm and cartridge manufacturers all are exact in their manufacturing processes. Most probably not the case, and that's where fine tuning the tonearm comes in. This last part is not at all easy with the stock Rega mounting.