Gene, I think without putting the motor on a dyno, you are really going to have to go by spark plug color and seat of the pants feel. I always move in either direction in increments of two jet sizes until I find the right 'zone' and then jet to within one number. A 100 cfm carburator increase on the same motor may not even require more fuel because it really depends upon the 'air pumps' (engine) displacement and how much the motor is asking for. If the carb change is the only upgrade, then the engine should be asking for the same amount of air. If it worked well on the 750, then go with the same jet sizes on the 850.
Get a baseline feel for how the engine performs, and then expirement with larger or smaller jets. Plug color is always a good indicator...I always shoot for a dark to medium brown color on the center electrode. Good luck and hope this helps some.
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