You don't need that R25 from potentiometer wiper to ground, the LED in the optoisolator will provide a similar quasi-log effect when being fed from an appropriate potentiometer.
Here's how to work this out:
The opto LED has a forward drop of about 1.7V (will vary with current). When the pot is cranked full-on, it puts 5V on the wiper. You need a series resistor to limit the LED current to (say) 20 mA. This value will be R = E / I, or (5V - 1.7V) / 20mA, or about 165 Ohms. This is R3 in my schematic, I used 180 Ohms.
To get a quasi-log effect, your potentiometer value has to be significantly greater than R3. If the value is too small you burn excess current (and perhaps the pot), and the log effect will be minimal, and if it is too high the log effect will be to extreme. I chose a 5K pot, this might be OK.
Since the LED won't turn at all much below 1.7V, to avoid a big "deadband" at the bottom range of the pot, put a series resistor between pot and ground (R2 in my schematic). 1.2K here sets the bottom of the pot at about 1V.
With these values, the LED current will range from 0 to about 20 mA (roughly), and at 50% pot setting the current will be about 1.3 mA. This may give you what you are looking for. Please note that my numbers and values aren't exact, since I assume a fixed 1.75V LED voltage.
Problems with this approach:
There is significant current flowing through the pot wiper -- depending on the pot this may be an issue.
The opto specs don't provide transfer curve data so I don't know how it's actually going to behave.
The LED "off" voltage isn't provided, so you might need to adjust the value of R2.