[chrispedersen]

its better than adding.
2 5% chances > 1 10% chance, as you have the small possibility of cancelling two events.

Well, I meant "is better" by the > sign.

I grant that the math has been elegantly presented and is right. However, I wonder if there is a way to test this.
I raise the question for the following reasons:

If you mod 5 10% chances of a dominions magic random - they get summed into 1 50% chance. (Try it). I wonder if that same approach hasn't been done to fortune tellers.

The second reason I wonder is because around 20 fortune tellers, I don't seem to ever get bad random events.
If I get industrious.. I'll run a test on that.