This doesn't sound very scientific since there is really no way to no for sure what that curve looks like below the horizontal line (at 0). What you might try is where the curve hits that horizontal line, try to get reasonably close to the tangent line of the curve at that point. Do that at both points where the curve hits 0. Let the two tangent lines intersect each other below the 0-line. That should give you a "ballpark" idea as to how much is negative. That would also give you a worst-case scenario. I hope this has helped a little bit.
Also a question: that dotted line that goes from one peak to the other. What does that represent?