Delmar O'Donnell, it takes, as Jon F notes, a lot of testing, but the basics are pretty old news.
I use now 1/4" gap with a slow burning ion stove, probably in the same efficiency range as the flat cats, give or take a bit, well, probably a bit more efficient, but similar.
With a fast burning stove, you have to do more playing around with it, but one thing I'm fairly certain of is no amount of 1/4" holes at the bottom will be enough, since they weren't when testing a slow burning efficient stove, but if you cut out long slots or just make the screen stand on nubs with gaps between to form long continuous air entry points, you get very good burns and the best efficiency I have found so far. Since cat type stoves don't work particularly well on narrow pots, I don't test them very much, since I use a narrow pot. I might do some wide pot testing though just to see in the future, if I'm really bored or trying to avoid some even more boring work, or something, but my guess is, for the cat stoves: nubbed base for max and lowest entry point air inflow, 3/8" gap between pot/screen, 4" high screen, depending on how much room between pot and ground, 4" assumes 2" or so high stand/stove, shorten screen height if the gap is less. Though I'd start testing at 1/2" just to see re efficiencies and speed of 2 cup boils, but I am guessing it will be around 3/8" because of too much testing in the past. 1/4" is definitely around what you want for slow efficient stoves.
I'd have to really test a jim wood fancy feast to make sure, of course, my guess is you may need a bit more than 1/4" air gap on top, but I'd only really say that for sure after testing the stove/pot combo to see. Takes a while, the easiest is to a make a screen too big and then just test the various gaps / bottom air inlet sizes until you find the one that works best.
Also, if the screen is too tall, that will cause other very non intuitive issues, something only a lot of testing finally confirmed to me.