I personally would no longer go any deeper than when my arms hit a 90 degree angle when using any extra weight. Tore mine in the bottom of a deep dip with 100 pounds on. GREAT exercise but be careful
never really checked but it's probably below 90. I find if i don't go low, the emphasis is on the tri's more so than chest, which is not what i am looking for with dips.
I must be lucky (knock on wood). I am 47 and I bottom out every rep with no shoulder discomfort at all. I`ve always did them this way. Slow and controlled down and explode up. I keep my torso slightly bent forwards and my knees slightly bent up so my body * kinda * resembles the letter C . But these days I only use around 25/30 pounds so its not like I have 80 or 100 pounds around my waist. If I did that I would go down and stay down