You can't rely on the graph to show spl output or frequency response. Once you put the enclosure in the car it changes everything for car to car.
Now rule of thumb for building sub box. Usually for 1 12 inch sub you won't need more then 3 cu.ft of volume. And you don't want to tune it so low. Depending on what you listen to tuning at 35hz will do good all around as music don't peak any lower. But the lower you tube it the less output you will get above 40hz and above and that won't sound good for rock or anything with midbass.
And generally if you listen to hip hop, rap and r&b like I do with many more other genres. It will be pointless to focus anything above 60hz for sub stage.
Also a port can be straight unless the box is not long enough for desire port tuning. And generally we do 12-16 sq.in of port area time our box volume to get desire port area.
So if you have a box with 3 cu.ft of volume. And say the box is 16inch high. You (times) 3 by let say 14 sq.in per cube of volume. That's 42 sq.in of port area needed in a 3 cu.ft box. Now subtract wood material. Mdf I use is 3/4 inch or as a fraction is .75 of an inch. So the top and bottom of the bottom of a box 16 in high will leave 14.5 inch high for the box height. Divide our 42 sq.in of port area to the height of the port to get our width of the port to reach our goal of 42 in of port area that comes to 2.89 which we can round to 3 inch wide by 14.5 high port tune to what you want. I don't recommend lower the 33hz but you gotta make sure after you calculate the displacement of sub and port volume that you still have 3cuft left of net volume.
If you have an android device you can download Torres box calculator in the play store and get all your dimensions for you box. It's pointless to compare the output on winisd cause it's unreal and confusing. I used bass box pro 6. But honestly now I just get my desire box volume and port area and tuning and that all.
I will post pics of boxes I made