IMHO..... once you get to ten reps....start adding weight. Even if its only 5 or 10 lbs. Adding the weight (depending on how much weight you add) will bring your reps down to about 7 or 8 or even 6 reps and then start working back up to ten reps. Once you get back to ten reps again, add some more weight and work back up to ten reps again. etc. etc. I`m not sure why you would want to do 20 rep chins unless you have a specific or personal goal.