Midrange mixture is mostly controlled by a balance between the idle needle and the high-speed needle. To start out, set the idle mixture to stupid rich. The engine won't idle, so just start it at 1/4-throttle or so. Have the high-speed needle set to a very-rich setting. Start the engine.

Once it's running, advance the throttle to wide open and adjust the high-speed needle to just rich of peak RPM if the engine's had enough running time to accept it. Once it will run "all day" at this setting with little or no change in needle setting, then you can retard the throttle and look for the idle RPM and mixture. Any idle lower than 2,500 RPM or so is good. If you are trying for a lower idle and your engine starts having handling problems, then you have the wrong idle RPM. Trying for a too-low idle RPM is the source of many midrange problems.

One thing we hear a lot about is that the engine won't idle low enough to keep the plane from moving. That's not an engine problem per se, but a problem with the fact that you're on a very smooth surface, the wheels roll freely, and you likely have the wrong propeller on the model. If the airplane will fly the way you want it to fly, and will idle down enough to allow you a nice, slow landing, and the engine handles well, then you have the correct propeller. You will just have to accept that your model will just not hold still on the ground.