Carb CFM.. Increase?

IMO,
One of the reasons it's wrong is because , to the carb, a 408 is not a 408. The formula ignores the total chamber volume. If you have a 10/1 engine then the chamber size is 408/8/10-1=5.67cubic inches (~93cc), so the carb is seeing (408/8 plus 5.67)x8=453 ci of volume ,on that 408.
So then, that formula more or less works;
6400x453/3456=839cfm
I know it ain't 100% correct because at 8/1 Scr, the volume jumps to 7.286 cubic inches making the 408 into a 466, which would require a 944cfm carb...... which makes no sense.
The formula works off the swept volume, and is the best we have.
But again, IMO , that is where some of the error is.

For my 11/1Scr 367 it works pretty slick; {(367/8/11-1)plus 367/8}x8=404ci, and
404x6400/3456=748 cfm. And I love my 750DP.