I heard a lot of people saying that CDR is a good representative of real RC.
Just like you, since English is my second langauge, I was scoring around
12-14 with Achiever, and 18-20 with TopScore.
Since I never used CDR, I cannot stress how similar the program is, but
I heard CDR is really good to practice RC section.
On real DAT, I was able to get score 20.
If your reading is bad, try to solve only search/destroy types of questions.
Do not waste your time by solving tone/interference type of question.
Only go over when you have time to do them