With regards to progression, are you asking when it's best to add weight?
I was thinking more of a progression of the rep goal (for instance, 25 for the first few weeks, once I can do ten for the first set bump it up to 30, etc). Ofcourse it would over time turn into an hour-long chin specialization day which is not the goal of the program, so any alternatives are considered.
If I'm not mistaken(sorry I have no access to the books right now since I'm at my parents for holidays) Rippetoe suggests one should do at least 15 good chins in a set before adding weight, I guess this is a good approximation? My personal guess is that ensuring strong motorical competence in the movement is the "actual" requirement before adding weight, and in getting to 15 you sort of have to achieve familiarity with the exercise.
The reason I think a progression of the rep goal would be in order is that getting to 15 reps will take some time(I'm currently doing 5), and keeping the actual work done in one exercise at the "same level" in the meantime seems kind of counterintuitive when I'm hoping to add iron to the bar in all other exercises every time I train them.
I just came to think of an alternative to adding reps to the rep goal; I could eventually just add one max set(or even a ladder, though that may be overkill) after I complete the rep goal any time I complete it in four sets or less, and keep this structure until I get the 15 and can start adding weight.
Btw thanks again BendtheBar and everyone else for your time and help. I also hope you all have (had) a peaceful christmas/hanukkah/kwanzai/few days off work/insert alternative name for it here