You are coupling a semi-trailer to your tractor but have not yet backed under it. The trailer is at the right height if

Before backing the tractor under the trailer, adjust the landing gear to position the trailer at the proper height. The trailer should be just low enough that it will be raised slightly by the tractor when the tractor backs under it. If the trailer is too low, the tractor may hit and damage the trailer nose; if it's too high, it may not couple properly.