I don't actually think your butt is too far out of the saddle as far as up and down goes. It's just a little far forward.
I can tell you, in my experience (up to 1.40m), it's more dangerous to have too long of stirrups over a big jump than too short. You need to be significantly above the horse's back, to the point where the saddle hitting or touching your seat at all over the jump would be downright impossible. Your seat coming into contact with the horse's rounded back or the saddle over a big jump is a recipe for getting launched over the horse's head.
To fix being a bit far forward- as you approach the jump, don't think about going into two point. Think about allowing two point to happen as the horse comes up to you. Trotting verticals might help you learn to wait for the fence a bit.
That being said, your lower leg could use strength. It needs to stay glued right underneath you, and right now it's sliding back a bit. At take off, I purposely hug my calf to the horse and keep it that way. This will only work if your leg is strong enough. Trot around in two point to strengthen.
To prove my point- Have a look at this picture of Eric Lamaze. His leg is not slid back at all- it is a foundation underneath him. But ALSO- Look how high his seat is out of the saddle. It simply has to be that high to ensure that:
A) His seat doesn't interfere with the horse's back and form over the fence.
B) The horse's back doesn't interfere with his seat and position over the fence.
I chose this picture because it isn't from some equitation class- it is a practical photo of one of the best riders today getting a job done.
Don't lengthen your stirrups- strengthen your legs and wait for the fence.