You check the histogram of the blurred mask to make sure it covers the 0…-8 EV range as well as possible, so you can make changes to each brightness range separately.
If the histogram shows a narrow band, say, -5 to -3 EV, you won’t be able to make fine changes, as dragging the -3 EV point also drags along -4 EV and, to a lesser extent, -5 EV, to guarantee smoothness.
On the other hand, of the histogram shows that many values are over 0 EV or below -8 EV, you won’t be able to make direct adjustments for those; for everything above 0 EV, the setting of the 0 EV ‘band’ is used, and the situation is similar for those below -8 EV.
