What I'm trying to work out is when using auto bed levelling, if a 2x2 grid is supplied does it just use that as it's plane ie. no calculations required to try and establish the best plane to use? I understand that a 3x3 or 4x4 tries to calculate the best plane passing through all of these points, but a 2x2 grid is a flat plane...or are the planes calculated in triangles not squares?

Also, does auto-levelling adjust the X/Y co-orordinates of all subsequent layers depending upon the slope of the print bed plane. If I printed a tower on a really sloped print bed does it just try and correct the first layer (or few layers) to straighten, or would it produce a 'leaning tower of Pisa'?

More testing then I guess. I checked all of my rails to ensure they're parallel and evenly spaced...all seems fine there too.