That a very vague question. There are many scenarios to that question. A rough answer IMO would be 2 to 4 jet sizes. Your ajusted altitude changes with temp, humidity, and baro pressure. In my experiences, there is 1 1/2 jet size change for every 2000ft of air change. Make a pass with the 80s and try 78s and see what your MPH does. If it goes up stay with the 78s.. If I didn't have a air monitor and EGT. Use your MPH, I would keep leaning my jets until I got to the fastest MPH and then go up 2 jet sizes. Usually is pretty close.