Yes, the "bottom" or one side of the plate is the datum feature with a flatness spec of 0.75mm and parallelism spec of 0.15mm. Since the tolerance is greater with flatness and the product has high variablilty in flatness, it is difficult to rely on the surface plate as a true indication of the plane.
Otherwise Stijloor methods (which is what I have tried) is all I can come up with. But I feel my results are not accurate beacuse of the flatness.