If you don't have Greg Everett's book, get it! In the programming portion of the book he covers loading and volume. Due do multiple variables the %'s are just starting points but non the less prove to be beneficial. I don't have the text handy but I believe he based the RDL off of the Back Squat and for the rep range you mentioned above a good starting point would be 50-60%....adjust as needed....if I am wrong about what the book states, I apologize.
With that said, are we on the same page as to how the RDL is performed? There are a lot of good video's in the exercise section of this site. I am relatively strong in clean and snatch DL's, but I don't believe I good perform the RDL with 150% of my best clean while maintaining the proper mechanics for 8 -10 reps.