Good discussion.  But wait a minute here.  Isn't the average number of stars per galaxy also needed?  If it was only, say, 100 stars per galaxy, the total number of civs would clearly be a lot lower.   Or, let's see now - is it simply that star births per year times the age of a typical galaxy GIVES you the average # of stars per galaxy and so that number is irrelevant?  I guess that's it: and for the Milky Way,  with it's 13 billion year life, and current complement of 400 billion stars, the number of star births per year ends up being 30.  But with a smaller galaxy, like a dwarf, the number is probably closer to 3.