Matchbook is the same thickness, or a 12 pack box, or cereal box.
I's not critical anyway and could easily be set by eye, but the cardboard makes it easier because you don't have to keep the gap where you want it while tightening.
Just stick the paper between them, seat the coil on the paper and tighten it down then pull the paper out.
That big tapered screw is indeed the mainjet adjustment, but you probably already found that out.