Don't worry about too much cfm.. A vacuum secondary carb will only flow what it is asked to flow.. If you have done the math and discovered you only "need" 800 cfm, guess what? That's all your engine will draw.. and you guessed it, all your carbs will flow..
So what if you have 1200 or 1300 cfm you only need 800? You are about 32% over carburated. The only way to really know is to measure vacuum above the carbs on a dyno to measure any restriction optimum would be .5-1.0 hg of restriction, getting good flow without stagnating. If you have too much restriction you are losing power. If you have no restriction you are losing drivability and will feel a "bog". The reality is you are probably only opening your secondaries 65-70%..
Some really smart people
who run lots of cam and big valves (reducing restriction) and potentially killing bottom end opt for smaller carbs in order to increase restriction within the intake circut for better drivability, fuel mileage, lower emissions and no bog.. Mid range top end HP suffers, at 5500-6500 but drivability and idle are crisp. It might cost you 30 hp at 7000 rpm but might be worth it..
Done well a pair of 450 cfm carbs with #62 main jets will flow better 1500-5500 than a pair of 600 vacuum secondary units will.. You will have to be willing to live without a choke.. (just one more thing to mess with) Run them progerssive for really impressive fuel milage (try 15 mpg) or run them non progressive for a ride like you wouldn't believe..