If Black doesn't answer, would be White's privilege.
Black has 1 point less and White has 6 points more, so the threat is 7 points.
By playing the reverse sente, Black gets 1 point at a.
Also there's another potential point at b.
Since b is gote for both, there's 1/2 chance Black would get it.
Thus, the reverse sente worth 1.5 points.
The answer: White should make this move when the ambient temperature is between 1.5 and 3.5 points.