With Setup menu point C you define the (center) pulse width for the rudder servo. Almost all available rc servos work with 1500 - 1520 μs. But there are a few special rudder servos on the market which use a different center position pulse length.
Here you can find a list of parameters for the most common servos. Please understand that we can not list all servo types. If a servo needs a special pulse length this usually is mentioned in the data sheet of the servo, mentioned on the packaging or directly printed on the servo. Ask the manufacturer of the servos or your local dealer for detailed information. If in doubt about the center pulse for your servo use the setting 1520 μs. It is very likely that the servo will work with this pulse length. Also when the servo is rated with 1500 μs center pulse use this setting. There is barely any difference between 1500 and 1520 μs and the operating pulse range is nearly the same, so these servos are all of the same type.
There is a relationship between the setting of the rudder servo center pulse length and the (rudder frequency). For technical reason only special gyro servos with 760µs pulse width can be used with frequencies higher than 333Hz, as standard pulse width would be too long and would not fit into the pulse!