I had this problem as well. we have a metal door frame that was throwing the compass direction off by many degrees.
Obviously in ancient times they had no metal doors to worry about so....
My solution: I had a friend hold a string at the door and I stood outside the house, in front of the door, and lined the string up with the center of the compass. Kind of like a surveyor might do to get the appropriate alignment.
We checked this by also lining the compass up, outside the house, with the wall of the house that ran parallel to the door.
In the Diamond Hexagrams book, it specifically says to stand at the door frame and look outside and take the measurement there regardless if the metal throws the reading off.
However, in the Diamond Feng Shui Course it says to check the reading at the door AND three feet away to find what is consistent and look at this over several days as well.
This suggests to me that for Feng Shui you want a real, accurate reading not influenced by the metal in the door, which seems logical to apply to Hexagrams as well since you have to use that Mountain to begin with.
best wishes
