I have been working and pumping fairly successfully for the past seven months. Trenton is 10 months old. I also bf my dd for 8 1/2 months, and worked during that too. So, I know what you're facing. It isn't easy, and takes a lot of work, but it can be done.
I found that I had to pump one more time than Trenton was eating. To get enough for two feedings, I pumped three times. In the beginning I had plenty and was able to freeze some every day. Now, I'm finding that I have to pump at night before bed to get enough to fill his two bottles.
I have always allowed Trenton to nurse at night if he woke and was hungry.... This really seems to help stimulate my supply. Yes, I'm tired and would like to get past this, but bf is important to me, so I'm just doing what I can. On days that I'm home, I also feed Trenton a bit more frequently--again to stimulate my supply.
As for how much to put in his bottles--it's a bit of guessing. At first I allowed his providers to fill his bottles and determine how much he wanted. Now, I make his bottles, as solids are well established and it's generally the same amount every day.