RE: how do you calulate cfm's?
i don't know about where you are, but in nyc and surrounding counties, it's very regulated. the fire department checks the hood construction and the fire suppression system.
you need to know the btu's of the equipment under the hood, etc. whether or not you need a hood with make-up air. lots of details. your best bet is to find a local guy, ask for references, get various prices.
this is NOT a do-it-yourself job.
if it's not done right, and not done legally, and god forbid you have a fire, the insurance company may, and will, and will be totally justified, in denying any claims.