Hey Jesse,
which spindle do you intend to use?
My spindle manufacturer writes in the manual that the 80 mm spindle requires the clamping force be spread over 8000 mm² of surface. With 80 mm diameter = 40 mm radius this would give
$ bc -l
pi=4*a(1)
8000/(2*pi*40)
31.83098861837906715400
31.8 mm minimal height of spindle mount. Since Onefinity 80 mm mount is 2″ (51 mm) high this is enough (for this spindle).
Of course, the leverage force exerted on the Z rails and their linear bushings will be greatly reduced if distributed over a greater length, such as by stacking the two mounts on top of each other