A lot depends on how efficient your setup is at retaining CO2 in the water column. I'd start on about 1bps and go from there. On a 125g it'll probably take a few bps to get where you want.
Or go here:www.csd.net/~cgadd/aqua/articles.htm
You will see a link for downloading a 'desktop calculator'. This will tell you your ppm based on the numbers (kH/gH) you enter into it.
The calculator also doubles as a nutrient calculator for dosing macro elements.
Clay-- How you doing?? Didn't I send you some dwarf sag??
I always measure using the PH/KH method. Your bubbles per minute to reach about 30ppm can vary quite a bit, depending on.....
1. Your surface water movement.
2. Your reactor or diffuser.
3. The depth of you tank.
4. Plant and fish load.
ECT..