THE MATH:
377.6 cfm @.800
320 fps w/pitot
5.600 port length
280cc port volume
2.4 = constant
377.6 x 2.4 = 906.24 / 320 fps = 2.83in²
377.6 x 2.4 = 906.24 / 2.95in² = 307 fps
5.600 x 16.387 = 91.8 / 280cc = 3.05in² Avg CSA
5.600 x 16.387 = 91.8 / 3.05in² = 297 Avg FPS
