From my own experience, I was not very good at adding reps to my chin-ups. I tried using a rep goal and seeing how many sets it took me. I used ladders and I used to-failure sets. It wasn't until I added weight to them that I saw progress. I disagree with Ripp, I think heavy weight and low reps can be very benefitial to improving your chin-ups total.
I posted this write-up a while back. It really helped me get my chin-up total higher and it could easily be done in the context of the routine you posted. Can't do a lot of chin-ups?