The best voltage to use is generally two Volts above the regulated VCC voltage. If you are using a 3.3V Pro Mini then 5.3V would be best. If you are using a 5V Pro Min then 7V would be best. The higher the voltage the more power the regulator has to turn into heat and the hotter it gets. Nine Volts is generally O.K. and 12 Volts might cause overheating.