You're so fast! I believe 0,003 inches (aka, 3,0 mils) is the suggested tip radius for playback of a 77,92 (@50Hz) / 78,26 (@60Hz) rpm record. You might therefore want to make a truly coarse groove cut, since most "78" record players are set up for it being relatively wide, unless you can label it clearly as a hybrid, microgroove cut for 78 rpm angular V... Todd66 might be able to fashion coarse cutting jewels out of diamond. Might last longer if they have more bulk? (: If he can add the burnishing facets, you can have lower lacquer noise, but this is at the expense of slightly diminished treble... I haven't heard any smooth vinyl "78's," however... only slates, which I think involve gold-sputtering hot wax cuts for the Brugnatelli cell, rather than sugar-Silver shooting them.)
For microgroove cuts, the suggested pickup tip radius is now 0,000,7 inches, however, one cuts with much greater base width than 1,4 mils, normally. 2 mils is a good width for micro... especially if depth automation is included... But for "78s," you normally cut laterally and pick a more comfy width to seat and "channel" the pickup stylus.
I hear that Mr. H. cuts 10 mil wide grooves after repairing Westrex cutters as a test drive. If he's using a microgroove stylus, then that would make the depth equal 5 mils, leaving only ~2 mils of lacquer atop the Apollo's alu substrate - brinksmanship someone such as he can pull off safely. But, if he actually uses a coarse groove stylus, then it has an included angle of 110 degrees, which is a bit wider than the 90-degree included angle of the microgroove jewel, and this leaves more of a pearl-pulling buffer between the tip of the cutting stylus and the alu substrate. Surf's up!
If you ladel your own hot wax spinsters, you can make it thick enough that you don't care how wide you grow... ...plus, you get to spread and debubble the "soap" with a torch!
(Lead litharge, Montan wax, Stearin, Paraffin, Ozokerite, Barium sulfate, and soda ash...)