craig00:

There is an unfortunate tendency to be sloppy in units within a field of endeavor. The listed BTUs of ranges (and furnaces) are really BTU per hour. So 25 kBTU/hr is your range's equivalent heat value (using your conversion). BTU/hr is sometimes written BTUH.

The required fan flow depends on many factors, and the rule of thumb of 1 cfm per 100 BTU/hr is only an approximation for historical residential cooking styles (not cooking with gigantic quasi-commercial ranges having griddles and top broilers and what not. Issues affecting selection include:

How sensitive are you to smells in the house?
How far does the hood overlap the range top?
What is the height of the hood over the range top?
Is it wall mount (better capture) or island style?
How internally deep is the hood for containment?
Are you boiling water or wok cooking?
How large (in diameter) are the burners?
Electric produces less effluent than gas.
Electric coil and gas produce more effluent flow than induction.
Are you willing to install a make-up air system to accommodate a high cfm exhaust system?

(These are retorical questions as you have answered some of them directly or indirectly.)

Then there is the question of actual flow vs. rated flow. Fans are rated for cfm at zero static pressure, but this is not the condition that they are operated in. One has to oversize the fan to account for duct losses, hood baffle/filter losses, transition losses, negative house pressure, etc. (See Broan's fan curves on their website for examples of flow vs pressure loss.)

So, a true cfm of 250 would seem to be adequate from your conversion, implying maybe a 360 cfm or so roof fan. Judging from your actual fan use, you may not need more. Being more sensitive to odors, I would go higher for myself.

kas