getting my ad shown on only the most relevent content sites
That's pretty amusing! ;)
IMHO, start small with a few extremely targeted terms and see what you get. Since you cannot determine where on the content network your ads will be shown and have no control over those sites, you'll need to do your analysis of the results after they start coming in.
Once you get an idea of how widely dispersed your ads are, and how the content sites are performing for you, add a few more highly targeted terms and continue the process.
You could toss in a couple of less-targeted terms just for comparison.
