Range vent hood - 400cfm and stay in code or get a the 600cmf I want - heated make up air??

Hi
I am putting a new gas range and a vent to go with it. I am planing on an external blower. The good people at Broan mentioned that I will need at least a 400cfm to cover the BTUs that my range could potentially put off (55,000 or so) - so I'm thinking 600 should be better right? - ah but then I hear to stay within code for a 600 I need dedicated and maybe even heated make up air. Really? My house was build in the 70s and is far from air tight. The furnace is newer and has a fresh air intake in the basement. If I'm only cranking the vent fan to full blast on the rare occasion that I need to clear some smoke do I really need to go the trouble and expense of putting in make up air? - I'll be doing the install myself - no code police to worry about - just the wife!

Was thinking an external blower would be best to get the most power with the least amount of noise - true?

Thanks for the time.