Jim, how would you incorporate chin-ups to the basic 5/3/1 4-day routine if your goal was to increase 1 RM? Currently I'm about 95 kgs and my max chin (from straight hands to locking the bar to upper chest) is around 55-60 kgs. So far I've done them on the press day as a "1st or 2nd 5/3/1 movement" (what a bastard) and repping on them on other days.
Yes, from BW + external. For example 95 kg BW + 55 kg EW = 150 kg chin-up 1 RM. Then for example 90% from that would be 135 kg which equals to 40 kg external weight (135 kg minus BW) and so on. I think it goes quite correctly that way even though now my BW is around 60% of 1 RM.
I really feel chins get stalled more easily than other lifts. Might be the extra volume back gets from rowing etc. They're much more like deadlifts where at least I have to keep things fresh to keep from stagnating. I added 2,5 kgs (~5 pounds) like Jim recommends in upper body lifts.
I wouldn't really know where to start - chin-up strength isn't terribly important to me. I just do some chins and do them weighted whenever I feel like it. Because I had a athletic upbringing and physical fitness was important in my schooling, chin-ups were similar to learning the alphabet. In other words, it's not a big deal.
Perhaps others have more suggestions for you but I never found the chin-up to be anything more than a very basic assistance exercise. I've always been more concerned with squats and cleans. At least when for the first 15 years of training.