The plateau on this graph is hard to see. It levels out, then kind of stair-steps up. The optimum voltage was found using multiple counter plateauing and performance studies. The performance study can be seen below.
Here are the values for the above graph.
With not too many short pulses, giving the desired Gaussian distribution it looks like this is the optimum voltage. This study was run at 0.996 volts.
Note: This is the same study as shown for counter one in the multiple counter plateauing.