This is dangerously low. If your battery is in fact cranking at 11 volts and you have 8.5 on the coil that is a 2.5 volt loss. You have to remember when you are seeing this 8.5 volts the car is cold and the car is capable of starting at that time. You need to check it when it does not want to start. I will bet it is less. You could do two things. Wire a volt meter to the coil so you can monitor it in the car when it is acting up. Or make a jumper wire up to jump from the battery direct to the coil. You also could check the battery voltage when it is acting up on the hot no start. Pertronix needs a minimum of 8 volts or it is not going to fire.