how far from rear wall should proac response2.5 be

Your wall or my wall? Start somewhere and end when the distance from the wall meets sonic bliss with your ears. Easy enough? This is entirely dependant on your room acoustics, furniture, electronics-- everything.
I auditioned these some time ago in an audio shop. They were pulled out from the wall a good five feet from the rear and 3-4 feet from the side. They sounded great. I made them pull them back to what my wife would allow (about half that far) and they sounded dreadful. Lesson learned!
My Proac 3.8s are 7&1/2 feet from the back wall and they sound great. I have a large room (L shaped approx. 16 by 25) and I used the Cardas room placement guide. I also do not toe them in. Good luck!