Correct! The fastest progress came in the first two weeks. After that, I just estimated the values for the weighted chins. I feld the estimates were fairly accurate.
You're right. I guess that's the nice thing about being in high school, and having your basement as your gym. You get summers off, and the "gym" is just downstairs, so greasing the groove is a very convenient method.
However, if you have a chinup bar set up at home, along with the necessary weights, you could still partially perform the program, and get very decent results.
During the school year, I wanted to grease the groove (with a different excercise), so I did a set upon waking up, another set when coming home from school, one more set 1-2 hours after dinner, and a final set right before going to bed. Granted, I did not get the results I would have gotten had I included some 7 sets per day sessions, but the outcome was still very favourable.
Another time, I just didn't have time for even 4 sets (exam time), so I modified it further, and still got good results. I did 1 set every weekday, but instead of 50%, I used 70-80% of the max reps.
So if you don't have a chinup bar at home with the appropriate weights, I would recommend the latter approach. If you do, I would recommend the former.
I hope this helped.