Voltage to segments on 7 segment display?
According to the data sheet, pins 1 and 6 are both connected to the common cathode. The data sheet also specifies a segment drop of 2 volts at a current of 20 ma.  Therefore, you should chose a resistor to drop 3 volts at 20 ma for a 5 volt supply.  That resistance is 150 ohms. You may have to adjust this slightly since the data sheet values are only nominal.  However, there is a maximum specified current of 25 ma per segment which you should be careful to avoid exceeding.