Yeah, that's how I got it. If the absorption fractions are f1 and f2, the transmission fractions are 1-f1 and 1-f2 which you multiply and then apply the inverse map to turn it back into an absorption.
1 - (1-f1)(1-f2) = f1 + f2 - f1f2The corollary is that there's actually no such thing as 100% absorption but you can get as near as you want.
See it now, LOL.
I just look at how I implement it as a simulation on my spreadsheet. I tend to treat the problem as a series of layers or cells feeding light from one to the next in a chain.
Gives me a means to switch components in and out easily for simulating layers of atmosphere with differing characteristics and changing composition.