Additionally, it is exceptionally difficult to measure available demand because 

1. the web is so insanely fluid,
2. there are so many variables that drive demand (not just online but also offline and EVENTS etc.), and finally
3. because the data is so very not available. 

(These are also reasons why CLICK attribution to multiple campaign prior to conversion is such a thankless exercise. Implicit in that is the assumption of infinite demand, among other problems.)

I have PERSONALLY had a lot of success using Controlled EXPERIMENTATION techniques, such as, say, Media Mix Modeling, to understand both current available demand and also segment conversion effectiveness. And this has to be a continuous approach and not DISCREET. 

Additionally, it is exceptionally difficult to measure available demand because 

(These are also reasons why click attribution to multiple campaign prior to conversion is such a thankless exercise. Implicit in that is the assumption of infinite demand, among other problems.)

I have personally had a lot of success using Controlled Experimentation techniques, such as, say, Media Mix Modeling, to understand both current available demand and also segment conversion effectiveness. And this has to be a continuous approach and not discreet.