Take a look at the manual- most manufacturers give deflection at low, medium and high throws in mm. For dual rates, set rates set at 100% on the TX and adjust the throws mechanically by changing hole on the horn into which the clevis is installed and position of the clevis on pushrod. By chance if you cant obtain the medium throws at 100%, only then try to reduce the value down from 100% to get medium throws. Then reduce the low rate to obtain the required low deflection.

This method is conservative way of setting it up. Once you take off, if you cannot handle the medium rates, you can always go down to low rates. However, if you set up high and medium rates, you may be in trouble if you cannot handle even the medium rates.

Since you have the 6EX and it offers only dual rates, set it up with medium and low rates first. Once you are more used to the airplane, slowly increase low rates towards medium and medium rates towards 3D rates

Ameyam