RE: DL-50 engine
The idea is that the engine/prop combo will unload in the air... adjusting on the rich side of max rpm by 200-300 on the ground is a safety measure to ensure you will not be running too lean when unloading in the air. Personally, I don't consider my engine fine-tuned after adjusting on the ground. I adjust reactively to its performance in the air. too many variables differ between static on-ground tuning and what the engine is experiencing in flight.
Chad