Alright so I PM'd Torres and he had some great answers:
Torres:
physical port length isn't necessarily the length before it bends. that would only be part of the length. if a port is 4" wide by 10" tall and 20" long and doesn't bend, the physical port length would be 19.25". the missing .75" comes from the baffle or wall of the box that when added together gets you 20" long. now...when a port bends, the bend in the port makes it seem effectively longer which is why the physical port length is shorter. so using the same port, if it bent then the physical port length would be 15.25". you have 20" minus .75" for the baffle/wall, minus 4" due to the bend in the port, which gets you 15.25". here's a picture to kinda help show what i mean if you made the 'physical port' as long as the effective port (which is the port entered in the port length box).
so as you can see, if the port bends and it is still made the desired length then the port will effectively be longer than it should be, lowering tuning and taking up more volume in the box.
as for your box, the pic you posted looks good
and since you're using 2 common port walls in the middle, you're right that the width gets divided in half after the bend. so in the middle it'll be 4.8" then when it splits it'll go down to 2.4" and continue along the rest of the port.
hope i helped answer all your questions