I went down the rabbit hole of belt tensioning and found the above posts. I got in touch with LiamN and he provided some feedback which I highly appreciated. He recommended to write about my experience for the Shapeoko Pro XXL, so this is what this is.
First, I did not find it necessary to design / 3D print any additional stops. In the assembly manual for the Pro the procedure is actually to push the X rail to the front and work your way all the way to the back, whilst tightening the bolts. Thanks to the linear rails that seems to square things up nicely and the gap between the X rail and the Y end plates was the same on both sides.
On the front however there was a difference of about 1.5 mm which accordingly can only be caused by belt tension. The instructions in the assembly manual regarding belt tension are not very precise. So, I proceeded with the alignment steps in Liam’s tutorial.
I found the belt tensioning mechanism on the Pro to be very cumbersome. First, the bolts that hold the clips are rather short (my guess is, this was a cost decision to have all the same size for M5 bolts on the machine, rather than having to deal with different sizes.) So, as you are trying to get the bold in the thread, you have to put quite some force on the clip so the first thread is catching. But the next problem is that the belt is not that secure in those clips. If you tilt the clip even just by a little bit, as you try to catch the thread the teeth will slip. So next I tried to loosen the back and front. This helped with catching the thread, but there is a very fine line where teeth start to slip, if the tension is not high enough.
Next, I used an audio tuner app and the Gates app to determine the frequency. This took some more fiddling and required tensioning on the front and back, as the range is just at one single tooth in my case. In other words the frequency range as outlined in the tutorial can only be reached at one specific tooth. If you miss it by one, you will be either way too high or way too low.
I was very surprised how little a frequency change makes a difference in tension. and a how that difference in tension makes a big difference in the alignment of the X rail. I think, if there is so little wiggle room in the alignment, the tensioning system should be engineered better. I did see some mods that allow for better belt tension for the S3, but they require cutting a slot in the end plate. The end plate on the Pro is much thicker, so I am thinking about other solutions. There is a space of about an inch between the end plate and the idle roller, so maybe something could be fit there. If anyone did something in that regard, I’d be interested to hear about it.
Some additional thought: These are still the stock belts, that came originally with the machine. I do not know if they are enforced at all. Again, recommendations are welcome.