Manual leveling query

I have just upgraded my Cocoon Create Touch with Advi 4.0.6 and a BLTouch 3.1 probe using a custom mount which places the sensor tip about 3.1mm above the nozzle, right in the middle of the “2.3-4.3mm) range specified in the sensor manual. Everything loaded exactly as described and worked from the first turn-on. Very first thing I did was auto-level. Sweet! A credit to you, Sebastian!
After a few “playing around” prints, I started a little more serious tuning. Came to check manual leveling, and this is what happened.
The extruded moves to the center of the build plate, does a BLTouch cycle, then stops and says “adjust the thumbscrews until the feeler gauge is snug”. The user manual suggests a 0.1mm feeler gauge. Problem is, that the nozzle sits about 6.7mm above the build plate. If I jack the build plate up this far, I approach end of travel on my adjustment screws. And then if I repeat the operation, the nozzle will again stop 6.7mm above the plate at its new height, since (as I understand it) Zo is now defined by the sensor and not the absolute limit switch.
Do I need to worry about this? I fiddled with auto-level and after about 3 tries have a grid that is within 0.1mm in all 9 spots. Is that sufficient?

It looks like you have skipped some steps. Which guide have you followed? Have you performed the Z-height tuning?

Yes, I did the z height tuning first thing. It tells me the z offset is 2.52. I have repeated it just now a couple of times. Same result. (I have left the build plate at about the same position it was in prior to fitting the BLTouch, with only a bit of minor fiddling to get the auto level within 0.1mm). I am looking at the manual leveling screen as I write this, with the nozzle sitting well above the build plate.

The actual gap between buildplate and extruded tip is 6.65mm. The distance to the (retracted) sensor tip is 9.73mm. The z offset reported by the software is 2.52mm. I don’t know whether these numbers are helpful or significant. (I have used (inch) gauge blocks to determine these distances, so they are within about 0.03mm).

Oops, sorry, my mistake. I forgot to select a position from the keypad. Once I do that, it moves to a proper position, 0.1mm from the build plate.
Learning curve!
But that raises a follow on query - where is the thickness of the feeler gauge ever compensated for in terms of z-height?

It is not and it should not be.

Hmm…that seems counter-intuitive, to me, at least.
So, if I were to use a 0.2mm feeler gauge, my z offset would be 2.42mm instead of 2.52. In an extreme case, if I were to use a 2.52mm feeler gauge, I would end up with a z offset of zero - are you saying this would all cancel out somehow, somewhere and still all work ok? If so, what is the purpose of the z height procedure at all?
Can you perhaps point me at something published somewhere that describes how all this works?

You have to use the right size of feeler gauge and this is up to you. You should not use some random one. This is exactly the same than when you do not have a sensor and are doing only a manual bed leveling. You also do not specify the size of the gauge somewhere.

Presumably, the “right size” is 0.1mm, although the documentation is rather vague and non-prescriptive. The only reference I can find to this size is a parenthetical note in one place in the User Guide. Even so, it then establishes Z axis zero at 0.1mm above the buildplate. I expect that this may cause problems with first layer, especially with a fine print, without some sort of compensation.

There is no right size. It depends of you bed, the kind of plastic, the temperature, etc. This is why I make no recommendation in the User Guide.

Only in the section of the LCD Touch Screen User Guide under “Manual Leveling” do you say “Adjust each thumb nut so a feeler gauge (0.1 mm) can move between the nozzle and the bed with just a tiny bit of friction”. Another reference in “adding A BLTouch Sensor” says " Personally, I am using a 0.1mm gauge." So I agree that these are not prescriptive. But the second part of my question
“it then establishes Z axis zero at 0.1mm above the buildplate. I expect that this may cause problems with first layer, especially with a fine print, without some sort of compensation”
remains.
And if it depends on all those things, then where can I find some guidance on how to determine a “right size”.
Sorry to seem so obtuse; I am relatively new to this. Surely these questions have been asked and answered before, or am I missing something REALLY basic?

Apparently you think that there should be no space at all between the bed and the nozzle. If it is the case, no filament will be able to flow.

I don’t think that at all! What I do suppose, tho, is that the software needs to “know” where zero is, so that it can act accordingly.
If I set my first layer thickness at 0.2mm, the display tells me that this layer is extruded at z=0.2mm, as I would expect. But if the buildplate is actually -0.1mm or so below where the software thinks zero is, then the actual gap is larger and I don’t suppose the result will be quite as expected.

This is the way the stock firmware works, the way Marlin works. So it is the same for ADVi3++. Unless you have a good reference explaining that we are all making it wrong, I will not change the behavior of ADVi3++.

Of course I’m not suggesting that all the millions of users have been wrong for all these years!! ALL I am saying is that I, as a relative newcomer, don’t quite understand how z setting depends on (to use your words) " bed, the kind of plastic, the temperature, etc" and was hoping someone could explain it to me. I have spent many hours googling…

There are many factors that interfere in a non-linear way. So it is not something you will “understand” and find with a search. You have to try and adjust parameters empirically most of the time. With time, you will “know” how your printer is printing and how the filament you use is reacting.

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