I wouldn't want to put in a lean flight with an OS 40 FP. I have 3 of these and they're great engines. Pretty powerful for their size, but these engines don't tolerate lean runs very well. Setting it too lean while adjusting, then backing off again won't hurt it, just don't run it for 2 or 3 minutes at a time that way.

I've always found the easiest way to test for proper settingis to lift the nose of the plane. ie, set the engine to what you think is right based on engine sound or rpm, then lift the nose to near vertical. As long as the rpm increases, your setting is okay. If there's no increase in rpm, or the rpm decreases, your too lean. (Same as the pinch test actually.)
Dennis-