The optimum voltage should be within the first four points of the plateau. The plateau starts about 0.904 volts on this graph. The optimum voltage is about 1.124 volts which is within those four points.
The coincidence for counters three and six was measured twice. The frequency was 925 and 900 counts/minute. The average is 913 which is what the counts per minute for the coincidence of counters three, six, and seven was divided by.
Note: Counter three is the new counter three, reassembled with a new PMT.
In this performance study you can see that it has the desired distribution. A higher voltage gives mores noise, or short pulses, and a lower voltage simply lowers the number of pulses. This performance study was run at 1.021 volts.