What is the minimum distance from drain to edge? Can these be trimmed to accommodate a drain that is not centered?

Yes these bases can be trimmed more on one side than another to fit a drain that is off center. On a couple of the sizes there is already an offset option. The slope is the same all the way to the drain so there really is not a minimum required to be left, although you would not want it to be less than the cutout in the substrate for the drain assembly. Please give us a call to let us know what your plan is and we can recommend the right size base to fit your needs. Also see the video on the product page to show you how to cut the base and reapply the lip on the cut edges.