I only have a few minutes right now to comment, but capture and containment of cooking plumes requires a different approach to calculation -- one that you can find described here in multiple hood threads, including briefly in one likely still on this page. Also, in many municipalities, deliberate additional make-up air beyond that provided by house leakage is required above the few-hundred CFM levels. I can return to this later today.

Some more comments: There are three common approaches to cooking plume capture and containment CFM estimation. One is used with commercial hoods and uses the length of the hood boundary (where open) as a parameter, one is based on the BTU capability of the cooktop as a parameter (this is somewhat related to the actual physical processes and is common in commercial practice), and the third indirectly uses the velocities of the plumes as a parameter. The latter approach is the one I prefer because it works for me, and is the one Greenheck also touts in their most excellent guide found at:

http://www.greenheck.com/media/pdf/otherinfo/KVSApplDesign_catalog.pdf

Your calculation result, even though based on BTU capability, seems to be disconnected from your other values that I suppose are intended to incorporate room clearing and oven plume capture. (I'm not sure where your source for that value may be found.) In any case, wouldn't you include those two values in your sum?

Cutting to the chase, lets use 90 CFM per square foot of hood entry aperture, providing a 90 ft/min air velocity. This is one-half the nominal peak plume velocity for gas cooking, and when this air flow reaches the baffles the velocity at the baffle slots will roughly double, entraining the plume successfully.

By the way, capture success requires a sufficiently overlapping hood to intercept the expanding cooking plumes, while containment success requires sufficient velocity to keep the plumes from reflecting back out of the hood.

90 CFM x the hood aperture is the recommended air flow volumetric rate for plume containment. What about room air changes. The containment flow should be sufficient for that also, but one can always go higher given an open plan house. There is a cost, however. What about oven opening after broiling meat: Not all of this will be captured, even from a range under the hood, unless the hood extends outward and to the sides by a considerable extent. The rest of the broiling effluent will escape into the room and not dissipate until the room air replacement multiple has driven the concentration down to a low value. In that time some grease will have condensed on walls and ceiling.

Having a recommended volumetric flow rate is only part of the design process. Blowers are rated as having a flow rate equal to that when hanging in free air. The so-called fan curve shows what happens when there is some pressure loss. To truly know what the hood, blower, and ducting will do (windows open, no screens), it is necessary to have a fan curve for the blower, and the pressure loss of the ducting (calculable) and for the baffles (some data exist). The blower rated CFM has to be such that when derated by the pressure loss via the fan curve the result is the flow rate one wants.

But wait, there's more. In most locales, cooking with unscreened windows open is not desirable, and in any case, imposed codes require make-up air (MUA) be provided to keep the house pressure close to the outside pressure. This requires a controlled opening somewhere (at a minimum) and if heating and filtering are included, yet another blower. This MUA blower has to supply a flow rate given its own pressure losses equal to that actually achieved by the hood, less whatever house leakage is likely with the interior pressure only slightly low relative to outside.

So, it would be best if the details of the intended (or at least candidate) hood are provided.

Thanks so much for the explanation. I just took the higher number of my calculations per 2-3 estimations I googled on.

So far these are some I am considering -

http://www.koberangehoods.com/product/ch22-sqb-5-series/ - 640 CFM

https://www.homedepot.com/p/Broan-Elite-E64000-30-in-Convertible-Under-Cabinet-Range-Hood-with-Light-in-Stainless-Steel-E6430SS/202042620

http://www.zlinekitchen.com/under-cabinet-617

https://www.homedepot.com/p/GE-Profile-30-in-Convertible-Under-the-Cabinet-Range-Hood-with-LED-Light-in-Stainless-Steel-PVX7300SJSS/207189298

https://www.homedepot.com/p/ZLINE-Kitchen-and-Bath-ZLINE-30-in-530-CFM-Under-Cabinet-Range-Hood-in-Stainless-Steel-617-30/206922237?MERCH=REC--NavPLPHorizontal1_rr--NA--206922237--N

Generally, I don't promote hoods I haven't owned. I can only relate that it is desirable that the hood overlap the cooktop both side-to-side and front-to back. Side cabinets are partly helpful when the width has to be the same as the cooktop. Height (internal volume) helps. Baffles are generally better than mesh filters unless one is operating underpowered and can't meet a reasonable air velocity.

A 5 sq ft aperture would be best matched by a 450 CFM flow rate, and this would likely require a 675 CFM rated blower. As the aperture of several of these examples looks like it is less than 5 sq ft, they may have enough flow, but may not have enough cooktop overlap. In such a case, greasy or odoriferous cooking can be kept on the back burners.

Depending on your location, above 400 CFM (usually the rated, not actual value) requires some MUA scheme. A passive damper in an 8-inch duct into the house somewhere may be sufficient to be accepted by the code enforcement officer. There are at least as many MUA approaches as hood and blower approaches.

Also in general, there are significant cost differences between inadequate, barely adequate, adequate, and superb, and without some definition of your required objective performance, I am hesitant to suggest any expensive components for the overall ventilation configuration.

Thanks for the info on the filters. Unfortunately our back burners are small, so when we deep fry stuff the bigger burners are better. I have not mentioned but I would like if possible to get a quiet range-hood as well. If you have recommendations that will be great. is wayfair.com a good site to buy range-hoods? I am hesitant as some of the brands like "Chef" are made in China, so not sure about the quality. This is on sale - https://www.wayfair.com/appliances/pdp/chef-30-860-cfm-ducted-under-cabinet-range-hood-chfe1000.html . Correct me if I am wrong , so it sounds like for me to get a good coverage or overlap, I should probably think about a range-hood where the width is greater than the width of the gas-range? This may then require cutting part of my cabinet which I dont want. I would rather try to fit in the existing space. With that in mind, for a 29.75 - 30inch wide cooktop, do you have any recommendations(brands)?