Some kind of carpenter you are !
Just kidding my friend. The mistake lies in this:
" = inch
' = feet
So you meant 20' instead of 20"
and
1 foot = 12 inch.
Stupid imperial system.
Only 1 country in the world still use it....
I just checked the width of the T-molding on my Japanese built cab and it's exactly 15 mm. so that's fine.
The order is final at 120 feet.