Generally 30" above the cooktop is the recomended height. Don't bother with a 120 CFM hood - you should get at least 300 CFM and ideally 600 CFM depending on what kind of cooktop you're getting.

the recommended height on my 30" electric cooktop is 27"-30". measure what you have now, or at a friend's home and imagine yourself ladling out soup from a stock pot, or the height of the tallest member of your family using the cooktop/range to see what is most comfortable. my old vent hung at 23" and the top of my ladle would always hit the vent edge. i'm going with 30" this time around. i do agree with the post above about cfm. anything under 300 just doesn't seem to be worth it, in a new install. surely you can get a stronger one for under $500. i got a kobe, which gets positive reviews on GW. they're made in japan and very reasonably priced.

OMG, I'd love to see someone trying to cook with only 18'' of clearance between the range and the hood!

Like everything else in ventilation there is no single correct answer. Under certain circumstances you may elect to go higher than the standard 30'' recommendation--if, for example, one of the cooks is 6'3'' tall and would otherwise constantly hit his head on the edge of the hood.

You can compensate for increased hood height wtih a corresponding increase in capture area: in other words, use a wider, deeper hood. The definitive discussion of capture area and effluent diffusion was posted by kaseki on an earlier thread.

Here is a link that might be useful: help with ventilation (with kaseki's posts)

The short answer is: Yes there are formulas. I have posted the references in some threads (maybe now timed-out) in this forum. Search online for "Thermal plumes of kitchen appliances." Rules of thumb can be found in the ASHRAE HVAC Application handbook, Greenheck's website, and Wolf's website, to name a few. They don't necessarily agree, and most are intended for commercial application.

In any case, you may not like their implications re your present hood plan. But like audio equipment, significant improvements cost exponentially more significant funds.

A much longer answer would require interrogating you on cooking style, range type, sensitivity to odor, tolerance for grease condensation on walls, etc.

The two key concepts are capture and containment (which implies removal). Higher mounting makes capture harder while improving visibility, lower makes capture easier but degrades visibility (and may lead to hardhat requirements).

Wider and longer make capture easier. Cfm and other hood features such as depth determine containment success. Cfm is not so important to capture because the air flow velocity drops very quickly as one measures farther away from the hood.

kas

good point, and people are finally catching on to buying "wider and longer" hoods. you may want to think about getting a hood a few inches wider than your cooktop. my 36" wide kobe hood will hang over my 30" electric cooktop.