The plateau appears to go from about 1.1 volts to 1.25 volts though it is hard to tell. Multiple counter plateauing gives a more prominent plateau.
These are the values for the graph above. Two seperate tests were made using only counter four.
The optimum voltage seems be around 1.15 volts, which is the value that this study was run at.
Note: This is the same study shown for counter four in the multiple counter plateauing.