HELOFAN,
There really is no rule about ratios of rpm, but many times the tail rotor tip speed is the same as the main rotor (when a conventional open tail rotor is used), so the ratio of the rpm is often the ratio of the diameters of the two rotors. Many rotors use a tip speed between 700 and 725 feet per second. For the EC-145 that turns out to be a 6.5 to 1 rpm ratio.