How does the auto-leveling feature work on the SD Press

This is my understanding of it.  The software calculates the equation of the plane that fits the 9 sampled points the best.  The a, b, & d values listed on the z-calibration screen are the coefficients for that plane (z=ax+by+d). 

Now, there are two ways they could be using that plane to compensate for the levelness of the bed.
1. Calculate an offset for only the z part of the coordinates for any given x,y.
2. Do a full translation of the coordinates so that the axes are reoriented to that plane. 

You may be thinking, “Why would it matter which method is used?”  Well, if it's done the first way, objects will not have a base (or top) that is square with its walls.  If it's the second,  the base would be square, but tall objects printed at the extremes of two edges of the bed (mainly the front) will not fit within the build volume as it gets taller. 

Which method is used can be determined by printing a typical test cube and checking for squareness of the base with its walls.  Since I don't have my printer yet to test it, I'm only guessing that it's the first method.  It's the simplest to compute, and there wouldn't be an issue with prints not fitting in the build volume that otherwise should.

If it turns out that it is indeed using the first method,  I believe we need a way to manually level the bed if we want.  From what I've seen in posts, it appears that the y-axis has a 2% slope on average.  So, a 100mm cube would be out of square by 2mm.  That may not be noticeable for figurines, etc., but it could be a problem for creating a functional part with tight tolerances. 

Of course, all this assumes that the z screw and guide rods are actually perpendicular to the x/y plane. 

As a final side note, I really hope they aren't using the rounded values that are displayed for the a,b, & d values on the z-calibration screen.  No matter which method they use, rounding errors using only 2 decimals could possibly be up to 1mm over the length/width of the bed, which wouldn't be good for adhesion at the outer edges.