Loss of Internal air space????

whoever told you that has no concept of air space.

most "normal size" subs will result in a loss of 0.04 - 0.05 for a 10" or 0.06 - 0.07 for a 12"

in the long run that's nothing... if you're desired box for a 10 is 0.66 cubes... and the driver is 0.06 cubes --- then just build your box to 0.72 cubes

of, you can just build it to 0.66 and not care -- like most people will do... 10% either way usually doesn't make a world of difference in small boxes. if you think you're box is too small -- throw a small amount of polyfilly, since you can get around a "15%" psuedo gain with some fill.

as far as how to figure it in.. just like i said -- u buy a sub that wants 1.00 cubes ... your driver is 0.06 cubes -- you build your box to be 1.06 cubes.

.4 is INSANELY high.

I've got 1200 watt 12" competition grade subs that weigh 40 pounds each, about 9.5 inches deep, and look like step stools... they have an internal displacement of 0.16 cubes... that's over twice the displacement of a normal 12. so 0.4 would be like... a tree stump inside your box.