After drilling the platter for a Noval tube, I finally made a fixture to hold the platters firmly, but gently, in the proper position for drilling:
The platter sits more-or-less flush with the surface, where credit-card plastic pads work fine. Thinner platters may require compliant padding.
The solid model has locating pips at ±50 mm from the center and airspace below the platter for the drill bit:
The 1.16 inch hole spacing matches the Sherline’s tooling plate. The center hole seemed like a Good Idea, although it has no purpose right now.
The OpenSCAD source code is the same as before; just set Layout = PlatterFixture; and it’ll produce the right thing.
The Smell of Molten Projects in the Morning 
Instead of a hole, one option would be to print a square projection about 1/4″ high that you could use an edge finder on to find the centerline of the platter when setting up the job.
I just jog around until the laser spot vanishes in a locating hole and get Close Enough in XY for what I need, although I like the idea of a definite edge in a visible location. A recessed square, rather than a projecting stud, seems less likely to attract errant tools.
My first pass at the locating holes turned out entirely too small: a 1 mm pit faded right into the top infill pattern!