Discounting specifics of your knee pad of choice, the amount of pressure while sitting with knees against a tree while in a saddle is directly related to:
1)The height of your tether
2)the length of your bridge
3)the relation of your knee to your hips
4) your anatomy, specifically the amount of tissue around your knees and knee injuries
The higher your tether is, the less force is being applied to the moment arm of the tether. IE, put your tether higher, and the rope will pull you less hard into the tree. The shorter a tether, the more pressure would be felt on your knees while sitting with your knees against the tree. This would be your macro-adjust.
The bridge can be adjusted along the same lines as the tether, lengthening to reduce pressure while sitting. I would use this as a micro-adjust.
Where are your hips relative to your knees when you are having pain? If your hips are hips are higher than your knees while sitting, this will increase the amount of pressure of the kneecap. Conversely, lowering the hips in relation to the knees while in a seated position will reduce pressure on the knees while sitting.
For people with more tissue around their knees, bigger legs, heavier set folks, especially in the back of the knee, sitting in a compressed knee position (low hips relative to knees) for a long time gets uncomfortable. These legs types would benefit from more time with hips higher than the knees while seated.
Hope this helps!!
