Thanks Gonzo,
That explains it perfectly. Does the same approach apply when wake vortex minima are used?
E.g. say you have a heavy with a medium following. The wake vortex sep. requirement is 5 NM.
Do you then add a 0.5 NM buffer on top to give a separation of 5.5 NM when both aircraft are on the approach prior to 4 DME, to take into account the compression when the heavy passes 4 DME and begins to slow down?
Much obliged for your help!