If you really want to measure it you could also use a metric allen wrench in 2mm or 3mm.

Actually, I just read your original post again and it sank in a little more. Personally, I'd back the idle screw all the way off so the barrell closes completely. The adjust the 2-3mm idle gasp with the linkage. Set it up so the barrell fully closes when you pull the throttle trim all the way back to kill the engine.

The gap will just be a starting point. Adjust it as necessary to get the desired idle speed.