BEagle,
i think i've mis-lead you a bit.
when planning, it was advised that you look 5nm either side of track, not the MEF as per vfr chart. - I agree that this does take the unmarked into account, but can be very limiting if you are to fly miles from the cause of the problem.
take 5nm either side of track, identify highest point. if its a mast, great, if not add the 300, that all I'm saying.