Here is an alternative.
I have added two spline circles, where the 0.25 or 0.15 point would be.
By moving the Spherical-Falloff, you can place one circle over the left and the other over the right clone. While the center with 1.0 is over the front clone. (To be clear, moving the circle alone has no effect.)
Color remapping is off; otherwise, the color remapping changes the values.
Another point an 0.15 or 0.25 value signals (rightfully) for an alpha channel, a linear value space (not gamma). If you need the equivalent of gamma, change the entry in the Filter channel accordingly.