Morning. Some of this I'm sure you know. The K factor is from the manufactures engineering publications. It's the free area of a grill/diffuser. So if your diffuser was 6 x 30, and the K factor was .75 the actual area would be 6x30 = 180 sq in. /144 = 1.25 sqft, *.75= .94sqft free area.
If you require 500 cfm you will need 500 cfm/.94 area = 531.9 fpm velocity.
Most grill applications consider cfm required for the load and velocity based on noise levels and on how the room is being used. The recommended velocities are published in a number of books.
Of course the air has to be there so it begins at the equipment and duct design stage. So with your problem the size will depend on cfm required and the noise level desired.
Cfm=area x velocity velocity = cfm /area area = cfm/ velocity
I should have played the g'tar on the MTV. MK
You can be anything you want......As long as you don't suck at it.