Mathematically, the ratio is whatever Cp/Cd equals. That number includes all forms of parry and dodge that are affected by DR. Some guides try to give it in terms of total percentages, but that is harder to nail a specific ratio for since total percentages include the dodge and parry percentages that are not affected by DR. My guess is the 1:2 is based on a total percentage perspective rather than DR parry compared to DR dodge. If you are comparing DR parry to DR dodge, the ratio is always Cp/Cd.
EDIT: just for clarification, if x is avoidance pre DR and y(x) is avoidance post DR such that:
y(x) = 1/(1/C + k/x)
Then y_parry(x)/y_dodge(x) should be Cp/Cd
Where Cp and Cd are the DR constants for parry and dodge, respectively