Ok first off....bookmark this page, this should be the only box calculator you ever need:
https://www.the12volt.com/caraudio/boxcalcs.asp
I swear by that box calc, its dead on with Winisd and Bass Box Pro.
Moving on....
With the dimensions you have chosen 26 wide 18 high 18 deep and 3/4 thick wood we have 3.86003 cubic feet BEFORE sub, port and bracing displacements are subtracted. This number is often referred to as GROSS volume. After you subtract sub, port and bracing displacements the resulting final volume is often referred to as NET volume. It's important to pay attention to this when reviewing a manufacturer's suggested design.
Using that link you provided I see that the driver displacement is .109 cubic feet.
As far as tuning, 25hz is too low for a car because cabin gain boosts the low end. The lowest I would tune for daily is about 30hz. So lets calculate the port length needed for 30hz.
This is where it gets a little tricky. We need to know how long of a port to use, which will also determine the volume of the box. Since both of those values dependent on one another it might seem like a revolving wheel and can't be done. A program like Bass Box Pro or Winisd can figure this out real quick, but when you don't have those programs or understand how to use them, this is where that link at the beginning becomes real handy.
We know there is 3.75103 cf left after the sub displacement is subtracted. Now we will estimate the port displacement around .05 and subtract that from 3.75103 and plug that number into the round port calculator at the top of that link. 4 inch port, 3.7cf and we want a tune of 30hz. This calculates 7.24 or round up a tenth to 7.25 inches. If you are using a flared port the manufacturer typically recommends adding 1 inch to the port length to compensate for the flares. So now we are up to 8.25 inches of port length.
Now scroll to the bottom of that page to the volume calculator of a cylinder. This is what we use to calculate port volume. Keep in mind we want to measure the OUTSIDE diameter of the port, not the inner. A typical 4 inch aeroport is about 4.25-4.5 in outer diameter. Lets go with 4.25 for that number and 8.25 inches for height of the cylinder. Put in a 0 for thickness so the clac doesn't treat it as an enclosure, the result is .06568 cubic feet. Keep in mind the flares are bigger than a straight tube that we calculated from but the difference isn't important enough to make a difference and for the sake of simple explanation we will ignore that.
Our original estimate of .05 wasn't too far off. Go back to our volume after sub displacement of 3.75103 and subtract our port volume to get a final volume of 3.68545.
We now have all of the numbers we need to get final volume and tune of our box. Go back up to the top of the page and plug all the numbers into the round port calculator.
4 inch port, 3.68545 cf, 30hz gets 7.284inches (+1 for the flares) is 8.284. It's easier to round down to an easier fraction so 8.25inches.
So with those dimensions and a 4 inch aero port 8.25 inches long, you'll be tuned to 30.05hz.
When you are a beginner designing boxes, its easiest to use whole or half numbers for dimensions. The ports can get a little more specific, but I usually only bother within .25 inch.