First off...like others, I feel professionally & ethically bound to advise you not to experiment on yourself. You didn't ask, I know, but please allow me to get that caution out of my system anyway.
As far as the depth of needling goes, 1-1.5 cun can be a very small distance on a person with a small frame. It's a matter of professional judgement. Within the accepted range I needle until I feel the "Qi". You can be on the right spot but still miss the "Qi" by going too deep or too shallow or being off the point. There is an acute sensitivity that develops after treating hundreds of patients. So, do-it-yourself needling is not recommended for many reasons from practical to ethical to spiritual. If you're hellbent on self treatment, I would recommend self acupressure. The needle is less forgiving than acupressure if you don't get the spot (or the theory) exactly correct and can be extremely effective. Or better yet--make an appointment with a practitioner that you trust & can work with. Knee & back pain tend to be chronic conditions which require regular & creative treatment.
Regarding making up your own point combinations--yes, you could do that and you might have some success. Recipe acupuncture works for some people. However, I find that the strongest & most effective treatments occur when I adhere to the classical theory. If you want to know more about the channels--you might want to pick up a book. It's like asking someone to describe quantum theory to you via email...too long, too involved to make it worth your while.