Really the answer is a rhetorical one as the base of a rotary table is not unlike a faceplate- or a lot of oth er things being cut or ground in the workshop. So one suggestion is a bit of brass shim which shows up differently when the cutter has cut just a shade lower than the complete depth of the work. Again, if sheer accuracy is unimportant you could hot glue a piece of plywood between the work and the table
Thinking along these lines, I wonder how few examples are shown of using shims between the jaws of a chuck and the work.
So really the answer is 'whatever you think suitable'
On another comment, or three, I'm delighted to learn that you have something that will extend your skills to dividing and graduating.
Thanks for all the good suggestions.
Tried both the parallel and shim (tounge depresspor) methods and both worked after head scratching on how to clamp all down. Clamps left barely enough room for the workpiece.
Would hot gluing a shim/work sandwich to the table withstand the cutting forces without additional clamps?
Unlikely that bonding will hold the part securely when milling, maybe OK if drilling only.
If you don't have space on the table for holding by using the Tee slots perhaps you may be able to use a sub-plate fixed to the rotary table that has tapped holes for holding parts.
Depend on the part shape.
Typically the problem arise with a part with a large flat surface on the bottom.
I found that 1/4" plywood or MDF the compressed wood dust boards found for little money at the box store are exceptionally parallel.
Someone suggested brass sheet, it is a great idea if one can afford it.
There is a sight called clickspring.com this guy makes clocks and the like, he uses a sacrificial support usually ally and super glues the component to it no clamps, you can then machine the whole profile in one go which is very handy at times, the component is released with a little heat and cleaned with solvent. the sight is well worth a visit the tutorials are very well put together and some of the methods would carry across to our obsession.