Rule of thumb is total BTU divided by 100 equals total CFM. That means if you had a range with 4 18k BTU burners, then you should plan for around 720CFM (18000 x 4 / 100). Of course you can just take the kBTU and multiply by 10 (18 x 4 x 10) to reach the same number which is easier math. That said, it's just a rule of thumb. Total CFM required depends on the type of hood you use, your install situation, and the type of cooking you do. Long duct runs reduces the net CFM rating of the hood so a 600CFM hood might only be moving 400CFM of air if there is a long duct run or many bends. If you do stirfry, indoor grilling, or heavy saute, that would up your spec a bit as well. Also, nearby windows and doors that are likely to be open during cooking can be a source of drafts that can carry smoke away so they need to be considered as well. Typically, get the most you can afford and can fit in the space. Sometimes getting a larger hood to increase capture area can actually do more than increasing the CFM. Some would argue that since I never have 2 burners going at the same time that it's ok to have a smaller hood which is reasonable but does reduce your buffer. IMHO the "rule of thumb" already accounts for less than full capacity usage since I doubt 900CFM would be enough if I had 4 full power saute or stirfry pans going. In other words, I would not advise going lower unless you think your usage will be less than "average". Good luck.

I don't think you will be happy with recirculation due to the smells. I know I had a recirculating set-up prior to the remodel and I never used it because it was so noisy and didn't seem to help. Have you thought of using a chimney type hood so the pipe could continue up through the ceiling? I agree that an exhaust port out the front of the house would not be a nice look.

My parents never listen to me and altho they have all the money they need to buy top notch materials, they don't (I guess that's why they have money they need :o) Anyway, they go with things like American Standard and Broan from the big box stores. "Dad, stop buying AS. You know their fixtures craze and discolor. You know Broan is so loud you won't use it." So their kitchen sink, bathroom sinks and tubs are crazing and the bathroom fans are so loud they won't use them. Mom DID listen and by a Toto Washlet tho :) If someone makes a quieter bathroom exhaust fan than Panasonic, I don't know who it is. Whisper Warm is what we have. 110cfm for my 10x11 bathroom and an 80cfm for hubby's 5x8 bathroom. The reason for the timer is so you won't forget they're on. That's how quiet they are. And they do a GREAT job of removing steam and rude odors! I don't know about the lighted versions or what ERV stands for. Funny story....when DH and a friend installed the one in DH's bathroom, they flipped the switch and everything was quiet. One of them said, "I don't think it's working". It was. It was just that quiet :)

@Louise Blay: You left out too many parameters to make a decisive argument either way. Let me explain. The down-draft, or pop-up side draft that pulls effluent sideways and then downwards, can be effective against very low velocity grease entrained plumes such as might be generated by low temperature cooking, bacon simmering (but only if proximate to the pop-up) and perhaps fish poaching. I believe I argued above and in other threads what the plume diversion issue is between the possible air flow into a slot and the upward momentum of a hot oil/grease cooking plume. On the other hand, a flush ceiling register that is smaller than the plumes that make it to the ceiling will not capture moist greasy plumes impinging on the ceiling outside the register area. Slotted versions may pull air along the ceiling, but this won't be fully effective against impingement or thermal condensation of moisture and grease particles. You want overlap and whatever amount of below baffle reservoir is feasible. There is an intermediate case you don't exclude, particularly by your use of "ceiling mounted." A hood mounted into the ceiling that has a base at 7 ft (2.1m) or even lower depending on chef head contact can be effective if the hood entry area is at least 8, better 10, inches (20 - 25 cm) larger than the boundary of possible pan edges on the cooktop. What are the joist cavity dimensions in your ceiling? Even with a flat ceiling scheme you need to use the joist cavity for installation. Please provide some insight into what you cook, tolerance for grease and odor, and how fanatical you might be with respect to hood visibility.