INVERTED SUB-DISPLACEMENT VS. CUBIC FOOTAGE

sup fockers,

when you run your subs inverted and they call for a minimum of 1.0 cubic foot min. for each by the factory specs, do you subtract the displacement of the sub from the 1.0 when running inverted. For example, 3 subs would require 3.0 cubic feet. The magnets run equal .25 cubic feet each (x3) so would a 2.25 cubic foot box work fine??

thanks.