For me, the 13'9" nose height came from the height of the 5th wheel from the ground. The nose of my M129A4 (with the other type frame than yours) now sits at 13'3" on the M931A2 with the Fontaine 5th wheel. The TSS trailer is about the same.
I didn't clarify in my previous post- 14' is max legal height in Texas. But...
If you are off the interstate and over 13'6", and you hit a low hanging sign or traffic light (that would have been cleared at 13'6), you get to pay for it! If you are 13'6" or under, they pay for your trailer damage. Most of where I take my stuff is off the interstate, so...
Cheers