318 mystery problem, somewhat solved!

The mechanical stop screw for the throttle plates will affect the IAC open %. If you open the throttle plates, the IAC will close to compensate, maintaining your specified RPM. 20% at warm idle is a pretty good target, but as long as your cold start and warm up aren't sending the IAC open all the way, I wouldn't sweat it too much. You might be able to dial out a bit of the return-to-idle stumble and stall though. I don't know what that system does to the IAC once the throttles are opened. If the ECU drives the IAC closed once you are off idle, and opens it again as you return to idle, a more stall proof baseline throttle setting (more open) may help. If the IAC remains at it's previous position, it won't help much. It's as easy as turning a screw, so why not play with it. Just keep track of what you do if you need to put it back to where it was before.