bed leveling with CNC-methods

I've recently used the same method as CNC hobbyists use to zero out their work-piece (see picture below), by using my printers nozzle as a probe that lights up an LED as soon as it touches a PCB.

That worked extremely well, however I am puzzled as to how it ended up working - by sheer logic I would need to subtract the Z-height at which I was calibrating, which in my case was at Z1.2, so I assumed I would have to G206 a Z1.2 in marlin to correct for that..

However that ended up being way too close to the bed (even though my printer has a Z0.30 start g-code) - what ended up working was Z0.15, which is half of the Z0.30 set by my printer.

I'm close to believing it was marlin always z-stop'ing that dictated this, but why wouldn't it be then 1.2 off the bed still?

Does anyone have a clue / theory how that ended up working?
Thanks so much for any ideas!

how many points on the bed did you measure?

Quote
MechaBits
how many points on the bed did you measure?

four total (each corner) - I ignored the middle as it's the result of them and I can't affect it directly, only with e.g. mesh-leveling.

Edited 1 time(s). Last edit at 01/14/2022 07:54PM by pickedtransmitted.

are you using 3 or 4 screws, either way 1.5 is about half your extrusion diam with a little squish, but the good thing is when its working you can forget about it & worry about something else, I start leveling my printer touching the bed, some z offset seems to raise things a little, but sometimes can be too close or too far, but either way I think it remembers where it was on its last jog