I believe the general rule of thumb is to have 1 cfm for every 100 btu

I too have a Bluestar 36" 6 burner with a 900 CFM hood.
Our hood is 36" so there is some smoke that escapes due to the reduced capture area.
At 42" you should be OK.

I just visited the BS factory and learned they make hoods. I plan to research them this week, but you might also want to look into them because they can be custom colored to match your range or rangetop.

Grilling meat might be smokey, but short of that you should be ok.

@avid: other custom hood makers can do the same, so don't be led into believing that only BS can match BS colors. Modern-Aire, my hoodmaker of choice, will apply a powder coat finish in any RAL color on the chart, for no upcharge. Best hoods on the market, bar none.

I think you'll be fine. I have a 1200 CFM Prestige in a 40 inch hood over a 30 inch Blue Star. I wanted to be sure I would have enough power -- and I do! I wish though that I had paid more attention to noise --