It’s at least as important to assess the cost of risk as it is to assess its likelihood.
To get at this, ask: What will being wrong cost you? Unpleasant, yes, but not as unpleasant as not having planned for it.
Follow with: What can I afford to lose — or, depending on where the negotiation is — give up? Walk away from? Hold out for?
The answers to these questions, together, will provide you a significant anchor throughout your negotiations.