does this all make sence?
 

hdc315 copper
28" w
21" h
20" d
port 19.5" x 4" = 78sqinch
total port length 22"
tuned to 39.78 hz
cuft 4.53 before sub displacement
cut list
top 28x20
bottom 28x20
left side 19.25x18.5
right side 19.5x18.5
back 28x19.5
front 23.35x19.5
port 14.5x19.5

gonna power with 1500rms
going to do sub up and port back

Assuming 3/4 wood thickness, your box internal should be about 5.53 cubes. Subtract 1.18 cubes for port displacement (4.75 x 19.5 x 22) gives you 4.35 cubes subtract sub woofer and internal braces displacement and you have your net internal volume which is less than your calculation...looks like you forgot the thickness of the port wall.