At CBC, Elliotte Friedman writes that the actual % of which salary can vary from season to season may be as high as 30%, but that no season can be lower than 60% of the highest.

The most extreme deal you could do in that situation would be something like 10-10-10-10-7-6-6. Significantly limits circumvention.—
James Mirtle (@mirtle) January 04, 2013

Mirtle has more on this in the Globe and Mail.