700 works for me.
I think you will find the width of a user's avatar also affects this. When you, jolinari and a few others participate in a thread, your 197 and 200 wide avatars cut down the width availabe for photos.
In other words, if you limit avatars to 150 wide then you can allow for 750 wide photos, or 200 wide avatars allow for 700 wide photos.
Having said all that, I'm happy with the 200 + 700 format.