I am sorry, I did not make myself clear. When I say do ZZ with the knees bend at 15 degree, it has to be done for weeks or months before the next increment to 20 degree and so far. That was what I meant by progressive sequence.
You see we stand on our feet with the knee in a straight position all the time. They cannot take the stress if we bend our knees all the sudden. In order for the knees to take the stress, we must work them in slowly by standing with the knees bend at the lowest degree without hurting or damaging the tissues of the knees.
The purpose of Zhan Zhuang is to strengthen the legs muscles and the knee caps. The knee caps hold all the upper body weight while the lower legs is acting like a tree root to lock them to the ground to hold the body in place. I would like to go into more details about Zhan Zhuang in the following posts. However, I still love to hear more of your interesting ideas and questions to test my basic knowledge about ZZ.
I know you are taken up Tai Ji. Did your teacher tell you to bend your knees all the time while you're do the movements. Actually, when you bend your knees while moving, you were doing dynamic ZZ instead of static ZZ.
When standing freely and doing the tai chi form, I am still at about the 15 degree mark. And that's after three years of good training! I have been dealing with a back and knee issue that I sustained previously from a car accident, though, so my progress has been somewhat slow.
I've found that, in addition to standing freely like a post, it helps me to stand against a wall. I usually try to slowly slide my back down the wall and get into deeper knee bends, then feel for a while--press my back against the wall, and when I feel comfortable at one level I see if I can drop a little lower. I totally agree that you should not rush the progress. Pain and discomfort are signs that something is wrong with the alignment.