i run 170's and set my saddle to 81cm from my bb as well. this has always proved to be a comfortable saddle height for me. i then was borrowing a friend's bike that had 165's. going by my rule of 81cm, i set the saddle and went for a ride. my saddle height did not feel right at all. it felt too short while riding. i then measured my saddle from the center of my pedal axle with the cranks parallel to my seat tube on my 170's bike. i reset my saddle on the 165's bike and it fit much better. as for your knees, i'm not sure it would reduce stress, but with shorter cranks you are making tighter, smaller circular movements... so perhaps that would help...