I've regularly had to calculate masses and column densities from CO isotopologues, as have many others. I found a textbooklike derivation in my old notes, so I figured I'd publish them.
I don't have time to nicely format things now, but the code, associated figures, and a 3-page tex document with equations are all here: https://github.com/keflavich/co_properties I'm pretty sure this all comes from a homework assignment in 2007 or 2008, so it's likely there are errors.