HOD reccommends completing R&S 6 by 8th grade. This is the way that it has worked for us, but I'm sure there are others who have ended up at different places with the English and it works too.
3rd grade - R&S 2 (Bigger)
4th grade - R&S 3 (Preparing)
5th grade - 1st half of R&S 4 (CTC)
6th grade - 2nd half of R&S 4 (RTR)
7th grade - R&S 5 (Rev to Rev)
8th grade - R&S 6 (MTM)
There are more dictation passages in Preparing. Not sure what to tell you to do until you get there - someone else will have to help you out on that one.

Kathleen