If you want to impose a cap, it should be based set based on full speed usage time. In the case of that 18Mbs tier they set it at 300GB. This means that the 10Mbs tier should be capped at 166GB and the 6Mbs one at 100GB. These caps provide the SAME usage time per tier before going over. The question of applying the same overage charges for all 3 tiers is a separate issue that can be argued either way.
