I subscribe to the mantra that a pony bottle is to help you make it through a CESA - period. Its a few extra breaths to survive at whatever SAC rate you want to use. For the deep specialty, I used my 19 pony as my redundant source at the 15' safety stop. I did 8 mins on it and when we surfaced, it still had 1000psi in it. So I expect that with a CESA to that depth in mind, and a stop until it runs out then surface, I ought to be inside the survive envelope.
I also subscribe to "if its not a PITA, you'll take it with you all the time and even forget you have it". The 19 doesn't change my weighting and I take it on every dive - except pool dives with classes; unless its for demo purposes.
FWIW, my consistent buddy dives the same rig. Pretty unlikely both of us will have a problem and need to dig into the pony simultaneously. So that's ~40cu of extra gas that we don't plan in our dive times. Either way, I think thats more than enough for me/us.