Just put in mine (wall mounted chimney hood with external vent) with 900CFM. I was leaning towards the 1200 CFM, but felt it might be overkill since its unlikely that I will be using all the burners simultaneously all the time. So far, it functions exceptionally well.
When I visiting a local high end appliance store, the salesperson told me that anything over 600 should be ok. I also read that a handily calculation is to add the total BTU for all burners from the cooktop and divide by 100 for the CFM requirement. I am sure that is just a rule of thumb, rather an a scientific measurement.
The most important aspects are:
1) Size of the hood should be at least the width of the range, if not slightly larger.
2) Deep 24" canopy
3) The backsplash material behind the stove needs to be non-flammable such as tile or stone
4) External venting is essential
5) Kitchen needs to have the ability to open up to the rest of the house somehow to reduce the air pressure differential due to the powerful hood. Lack of air pressure can lead to insufficient ignition and Carbon Monoxide generation. If not, try and add a makeup air mechanism into the kitchen or your furnace.
Adding a picture in case you are interested.
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Range and range hood combo
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