marcinsmok, You can start with 1 bps -2 bps. You have a decent amount of light there, and ultimately, that will be what determines how much co2 you will need.
You should invest in a drop checker. Really, it's the best way to read your co2 levels. No more testing, guessing, etc. The ppm is really more important then how many bbs. BBS could be really different in 2 identical tanks, depending on the diffusion method.
There was some talk here about Co2 refill in Manhattan a while back:
http://www.plantedtank.net/forums/ne...2-tell-me.html