The fixed distance marker is usually associated with the ILS glidepath transmitter and is about 300m from the beginning of the runway. It assumes a screen height of 50 feet across the beginning of the runway
excepting the case of displaced thresholds where it can be used at about the 300m mark from the start of LDA. (if you don't have Perf A - thats Landing Distance Available)
At the old Kai Tak rw13 it could sometimes be an interview without coffee
by the HKCAA if you ignored the displaced threshold but it could be argued it was better to be interviewed than seeing if your 747 floats at the other end..