British Columbia

It may be a problem, but it also alleviates a lot of the problems of having to have a group continually decide on a number on good and bad years which would become highly political. Even if it was a non partisan group, partisans have their way of fucking with that kind of thing anyway. Then you get a bad government and or group to just start saying 0% year after year.

A mostly predicable set value based off another value is less likely to get messed with, even if it has its own problem.