Originally Posted by ramo

The basic rule is (Mileage rounded up to nearest 100) * (fare class multiplier)

where the fare class multiplier can be:
- 2.0 (200%) for (most) business and first
- 1.0 (100%) for "normal" economy (and some business, like with AC)
- 0.25 (25%) for low economy

Whether a given fare class belongs to "normal" economy or low economy depends on the airline flown.