2(12ft 9in+8ft 1in) is the perimeter, but no allowance is made for doors or windows.
Perimeter 2×(20ft 10in)=40ft + 20in=41ft 8in.
Due allowance must made for door(s) and window(s).
If there's one doorway and it's 2ft 6in wide the border length is reduced to 39ft 2in.
No allowance has been made for window(s), because the window may be higher than the border, whereas a door cannot be higher than the border.