Room treatment can be addictive

For me, the addiction comes from hearing the improvement that something makes and wanting to go further in that direction. I've spent three years on room treatments and still have further to go. Like equipment, it takes time to evaluate and incrementally improve. The ROI is much greater in room treatment than equipment purchases. 

After I put up my first configuration of treatments (absorption and diffusion) I would run my RTA with Pink noise and walk around the room. As I heard the sound change, I would point the microphone in various directions to see which frequencies were changing in relation to what I was hearing. When I identified problem areas, I could decide what treatment was needed based on that, and assess the changes. Generally, areas where highs were peaking got more absorption, boomy areas get more diffusion. Sometimes it's some of both. 

Trust your ears.