If it were me I'd shoot for at least 1mm. this isn't really an area you want to hedge your bets.
how much meat did he take off the caliper? Could you chamfer one side with a file for a little more clearance?
sharpen the edge of the comstar into a cutting edge that way if there is any contact it will fly cut the caliper and clearance its self.
haha. no, dont do that. but If i was being a cheap ass Id ride to a rough bit of road at low speed and casually mash the brakes and see if anything happens. if it was gonna be catastrophic later it might leave a witness mark under those circumstances. still would prefer 1mm clear.
On various racing cars I've had there's been as little clearance as like 3mm between various parts and the wheel ID. The main downside is that when you get a piece of gravel or another foreign object stuck between whatever the part is and the wheel it tends to machine your wheel in half and cause a rather abrupt deflation that is worse than a normal deflation because your tire only has one effective side wall.
In your case, I have a hard time believing you'll ever see a piece of gravel stuck between the spokes and the caliper. And it would have to be something really stout to actually stop the wheel, those spokes are likely to self clearance first, not saying this is something you want to try out... Also, clearance is rather binary, either you have it or you don't. Unless you expect thermal expansion or the previously mentioned debris to be an issue.
Here's a wheel in my office that I turned into a clock that was sawed in half by a nut on the left rear upright lower going into turn 4 (no chicane) at Mid Ohio while going about 135mph. I surprisingly did not crash but it likely cost me pole position and a race win as I had gotten pole, reset the lap record, and won the race the day before in a convincing manner.
Seat foam - go to a carpet store and get (free left-over if available) "rebond" carpet padding. Glue with 3M 80 (home improvement store) and trim with electric carving knife (NOT the wife's - you can find them cheap on ebay). Build layers until you get the shape you want.
that turned out well. Are you in Canada or USA? I have used a closed cell kneeling pad you can get from Princess Auto in Canada. It isn't the dense stuff like the puzzle piece mats you can get its firm but also squishy. Hard to describe but like thick neoprene. I made a cafe seat and covered a piece of that with vinyl then attached it to the seat. It was 8 hr comfortable. I road from Moncton, NB to Brifdgewater NS all back roads, 1 stop for lunch and 8hrs total and my but was fine.
This is the one. When you squeeze it you'll see why it's so comfortable. 2 layers of that with a cover would be all day long comfort and still thin and sleek.
https://www.princessauto.com/en/detail/15-x-20-in-kneeling-pad/A-p8099442e