RE: Setting up an OS 1.60, cline, bisson pitts, 18x6w
You can use 5 or 10% nitro and forget the head shim. You don't have to lean the low end so much. The idle controls the midrange and it won't hurt your idle to be slightly rich. You can do the pinch test with a hemostat or needle nose pliers. You may end up with a lower idle if it is slightly rich also.
How much time do you have on the engine? Are you running it on a test stand or in the plane?
The engine would run better on a Bisson St4500 or OS BGX muffler.