Requiring certainty is kind of absurd. Even if I blow up your car, it could have blown up by itself a few minutes later.
Sure, and in that case you wouldn't have been liable... but that's not what happened, so you are. If you think it's unfair to be blamed for causing an explosion that might have happened anyway, there's an easy way to avoid that: don't blow stuff up.
Adjusting damages based on wild speculation about what might have happened is just desperate wishful thinking.
My own sense is that the "fair market value" of the loss comes the closest to what is actually fair. So if I can prove that you deprived me of a 1% chance at $1,000,000, I'm entitled to a $10,000 recovery.
Such proof would be impossible for anything like a client meeting or a job interview, of course: they're not basing their decision on a coin toss. I can only see this working in a narrow set of cases, involving something with a known probability structure that can't be replaced, like a ticket for a discontinued lottery.
We don't know how to measure rights yet, just as we once didn't know how to measure colors. But we can use them because we perceive them directly, just as we did with colors. If someone says "I believe I have a right to torture children for pleasure" or "The grass and the sky look the same color to me (under ordinary conditions)", all we can say is that they are either lying or somehow their perceptual mechanism is broken. It is impossible to convince a person that he does not see what he knows he does see.
If there were billions of people who interpreted colors differently, who didn't see some color differences we did and saw others we didn't, we could hardly call their perception "broken". At best we could call it different.
Likewise, it's awfully presumptuous to claim that you just happen to have perfect perception of rights when you're surrounded by people who perceive them differently. If anyone's perception is broken, what makes you so sure it's not yours?