That is exactly right....

Keep it simple.  Don't worry about ads.  Think of it this way:

Imagine there are 500 people building AdSense sites.  How many of them do you think will be doing analysis based on value per click, ads available, etc.

Answer based on the other training info you have seen available elsewhere.

For me the answer would be that I think the VAST majority of them would be doing that type of research.  

Now, how many of them have tested that research?

Very few?

So lets take this to the next level.  If you are the 10 or 20 people that aren't following the OVERLY available advice about how to conduct AdSense research do you think you can possibly have an advantage when you choose 1 topic that builds a site with 200 or 300 sub topics?  

Less competition, less scrutiny from the search engines, etc.  Many benefits.  Because even if there are less ads, there are still going to be advertisers available.  But you are more likely to get better rankings, and many other benefits.

At the end of the day you can do whatever process you like, however I am just making my current process known.

Thanks
M