I am builing a sub for a friend and need advice on porting. The sub is an Atlas 15. The internal box volume is 8ft3. I will be using two 4" flared precision ports and am looking to tune the box to about 22Hz. I do not really want to tune lower because the room the sub will be used in is huge (5000ft3) and tuning lower would probably cost too much output. My question is do I need to subtract the volume taken up by the ports, the woofer, and the amplifier when doing my calculations? I think the woofer displaces about .11ft3. The ports roughly .10ft3 each and the amplifier maybe .05-.06ft3. This will take me down to a net internal volume of about 7.65ft3. Is this a large enough difference to be concerned about when diong my calculations? Thanks for your help.