I have been practicing Chan by following the instructions in your Chan Handbook.
I use the full lotus position but my knee does not lie flat on the floor. I could force it by tightening my back and legs but would experience pain and couldn't sit as long as before as a result.
It is not bad that you could force it like that. If you wish you should keep it up and your leg(s) will fall flat even faster and touch the floor. The more pain or discomfort you are willing to endure, the quicker your leg muscles and tendons will loosen and relax.
On the other hand, the average practitioner does have another way to achieve the same result: just continue to sit. In time, your legs will naturally become more flexible as your Qi becomes stronger. Then the leg(s) will naturally drop and touch the floor. Don’t be bothered until then.