slot port length calculator

I'm unsure about some details about my port length or maybe I'm just thinking too much into it but the calculator asks for port width / port hight / box volume and hz I want tuned to. Ok this all makes sense to me except, first "do I need to subtract subwoofer displacement and port volume when figuring my box volume?". Second "is the port length measured from inside the box or from the face?" (I'm using 3/4 inch MDF board.) Third "do I need to calculate another way when two subwoofers are sharing the same port?"Gman

More info...I have two kicker L3 15" subs running on a kicker 1000.1 zx amp. Box currently built is 8.87 cubic ft total. I don't have the manual but I believe the two subs combined displacement is .32 cubic ft (.16 each) box hight is 21" /width is 44"/ depth is 20". (Measured from outside dimensions) Using 3/4 MDF. Port is 3" width/ 19.5" hight/ (9.75" depth ,measured from inside box) (measured from face 10.5) vehicle is 06 trailblazer roof and back hatch dynamated. If I believe I'm using the slot port length calculator correctly my box should be tuned at 33.88hz...no?Gman

And yes both subs share the port which is located in the center of the box between the subs (subs and port all face same direction)Gman