I don't think you can make that simple an equation, because a change in the unemployment rate in itself does not necessarily mean the EI benefits will be affected. It can vary greatly, according to the length of unemployment, who is suffering from unemployment.
On your point about the 1991-92 deficit, I'd like to point out that the economic structure back then was significantly different from what it is now. For instance, the unemployment rate was significantly higher in the early 1990s than it is now. So all of the things being equal, even if we were to go into an economic downturn, the impact on the EI benefits and the surplus or the deficit of the account would be way, way smaller, in my opinion.