What the R value to this operational amplifier isn't saturated?
Your bottom buffer drives the output to -13.33V.  The top buffer can drive it only 6.66V lower before hitting -20V.  The top buffer has a gain of two through the last stage, so it must be kept BELOW 3.33V.

The voltage divider with the potentiometer then has to keep the center tap node BELOW 3.33V.  That means the pot value has to be at LEAST 5 times the 1K value of the fixed resistor, or that node will be pulled too high.

There's no danger of saturating the output on the plus side, since it inverts and there's no way to give it a negative input in this circuit.