How do I calculate the length of a slot port that shares walls with the enclosure and requires end correction?

I designed and built a wedge shape slotted port subwoofer box for a 10"Rockford sub.My slot port has one extra wall, the rest of the walls it shares with the enclosure. My port goes along the bottom edge of the baffle.

I built the enclosure using the SBL calculated port length of 392mm for my target tune of 40.1hz. When the box was completed I ran an impedance sweep and my tune was way off @ 34hz. Claude determined that there is some degree off end correction that should be applied to the port length when a slotted port shares walls with the enclosure. I'm not certain how the math works but I trimmed the internal port wall down and finally got 39.9hz @ 282mm port length.

I don't know enough abouthow it works to know if there is a calculable end correction factor, or if you have to build the box and then impedance sweep to figure out where to go from there but it would be great to have a section where you can apply an end correction ratio to the port depth so the 3d model and cut list etc can be correct.