2.5nm is the radar minima. Normal practice is for 3nm spacing to 4DME to be applied, so that the compression that occurs is still legal in IMC (when the Tower cannot see you so cannot apply Reduced Separation in the Vicinity of the Aerodrome).
If we can see you out to 6.5nm, and there is a headwind, we can apply 2.5nm spacing to 4DME, the proviso that as the Tower are visual with the one crossing 4DME and slowing down, and the one at 6.5nm, the longitudinal separation can legally reduce below 2.5nm.