As a rule of thumb, whatever height you come up with, if it is between 1" and 6" then you go the next highest foot, if it is between 6" and 11" then take it to the next 1/2 foot mark.
Now let me explain better; actual measurements 12' 4" Then you need 13' minimum clearance. If 12' 10" then you need 13' 6" minimum clearance. I go the 14' just be sure.
You have to take into account that there may be a dip in the road under that obstacle you are trying to go under which means do to the length of the coach or TT you could actually be at 2 different angles. NOT so pretty in the end
So what you really care about is MINIMUM Clearance you need to travel under safely not actual height of your MH or TT.
Besides unless you get out and actually measure the clearance distance yourself then you are trusting they haven't repaved on top of the last pavement