As per the flight manual of the BO105CB-5, you have
At 100% N2, the engine shaft is rotating at 6,000 RPM. The main rotor rotates at 424 RPM. The tail rotor shaft coming out of the main gear box rotates at 2,452 RPM. After the intermediate gearbox, the tail rotor shaft rotates at 3,065 RPM and finally after the tail rotor gearbox, the final rotation speed is 2,219 RPM. You 5.24 ratio is between the main rotor speed and the final tail rotor speed (2219/424=5.233).
Hope this helps.
Cheers