With respect, Aztec Flyer is incorrect. The 25% (or 35%) bonus is calculated against base mileage, before multipliers. That means that for a short flight it is 25% of the minimum (600 or 800) and for longer flights, it is 25% of the actual flown mileage, whether flying discount Y or full fare F.

For example, a WOWBMI LHR-EDI fare would earn 300 miles plus 0.25 x 600 (minimum) = 150 for 450 miles total.