After doing this exercise, I have been able to experience greater relaxation at various muscular levels. I have also been able to sense relief from tense feeling in my facial bones.

I understand that relaxation found in one area can be carried over to areas which are tense. I often experience a ' catch' in my knees besides stiffness too sometimes.
My question is: How far down the perineal floor
extends from the pubic bone. To get a sensation, we slightly caress the thighs before continuing forward
with the breathing exercise.

While other diaphragms seems to have clearly marked areas, the area covered by the perineal diaphragm is
not clear to me. Although I have tried to carry the
relaxed sensation in the thighs down to the knees, I
have not been quite successful. Please advise.

The perineal floor is the area between the anus and the genitals. It does not extend below this area. The sensate-able area that it refers to goes outward into the hip sockets and then up to about the level of the pubic bone.

Areas below this would be simply extending your awareness to release other areas that are distinct from the perineal floor.