While a 42" width would be ideal, it is fine at 36" wide given the adjacent upper cabinets that will help corral the smoke and grease. I am much more concerned with the depth of 18.5". That is entirely inadequate. Rangetops tend to be 27" deep or more (including the knob control panel). Check what depth your model rangetop has. But if typical, it will mean there is about 10" that is not covered by that hood. Not only is it not covering the front burners for exhaust purposes, but there is a greater exposure of the flammable hood surround directly over the burners rather than over the control knobs.

I know it's not the answer you want to hear, but perhaps you should either get the cabinet surround re-done for the proper depth, or pull it out and buy a stainless steel hood that is 24" deep.

As to cfms, you said "rangetop" with no other information. I am going to assume it's gas because the majority of rangetops are gas. Also the majority of rangetops have more btus than cooktops do. Your minimum amount of cfms should be 600, but if you provide the make and model of your rangetop, we can give more precise advice.

What is the duct size that goes from the hood to the outside? It needs to be 8" or 10" diameter or your hood will be more noisy and less efficient due to trying to pull air through too small of an opening.

Once you establish the actual entry area of the hood with insert, then I recommend that the actual CFM be 90 ft/min times the area in square feet. This will ensure containment of captured plume components. Whether this is all of the plume components will depend on the size selected.

Rated CFM should be roughly 1.5X the actual CFM calculated as above. This assumes that sufficient make-up air (MUA) is available. MUA threads here should be reviewed to understand this topic.

Also, just for the basic grounding in the subject that would have been good to start with, read the first dozen or so pages of the Greenheck Guide. Commercial venting principles apply to residential conditions with some variances due to kitchen size and cooking practices.

https://www.tagengineering.ca/wp-content/uploads/2015/02/KVSApplDesign_catalog.pdf

Great discussion.

Thank you for the KVSA link.