Again, a "rule of thumb" ONLY! Rotors should be approx. 1/2 the thickness of the magnet. The "rule of thumb" difference though, is the magnetic properties of the steel the magnet is being mounted on! Let's say someone "sticks" you with some stainless...boasting "it'll never rust on you!". Grades of stainless range from completely non-magnetic to "pretty good" magnetic-wise. That's why so many people are using brake discs for rotors!
I always thought you could actually have someone who works in an injection mold machine shop (like I have), make a mold for some fairly large rotors (let's say 18-24"). Have them made from carbon fiber, with lots of ribs for strength, a good hub, and pockets for magnet assemblies. VERY light, almost indestructable, and could be made extremely accurately. Make pockets in it to fit other metal pockets from a highly magnetic flux material to mount the magnets in, fitting tightly on all four sides and the back of the magnet, leaving only 0.001 - 0.002" of an inch less depth so the magnet stuck out slightly further, but the neccessary size & thickness in the sides, top, and back for maximum flux flow. Mount the magnets in those, and then mount that assembly in the carbon fiber rotors!
Practically NO issue with location, balance, and the weight reduction would be a great help! Also, the pockets the magnets would be mounted in would allow maximum flux density all around the magnet. If a replacement would ever be needed, you can have exact replicas either already made, or make more much easier as you already know the dimensions.
Once you have the mold "dialed in" for sizing, shrinkage, etc., make a few thousand of the things! At 4000 pieces (there would be two used per genny), the $200,000 cost of the mold would come out to $100 per genny! Dont forget, that's with holes, pockets, and even tapped holes already molded into the rotors! A good mold should be good for at least 100,000 to 500,000 parts, which brings the cost WAY down! Example: 100,000 rotors = 50,000 gennys. That's $4.00 per genny plus material & handling. Oooooh! It might even get up to $10-$15 extra per genny at that volumn!