Reporting back:
GREAT success. I did the test as described with the LP/HP on the pink noise. I went round the room watching the SPL meter, listening... I felt like I was in Ghostbusters
I made marks on the walls (near the corners) where the meter really went up, and smaller marks where it was it was up but not so bad.
Redistributing the treatment I had (which was just sort of thrown in until I had time to do proper analysis) according to this method gave me an 8dB improvement in the targetted region! It also really really (!) smoothed things out throughout the entire spectrum. The room sounds soooo much better!
Maybe a more experienced person could do this without the added step of filtering the pink noise, but it doesn't take long and it has really helped me. Still more improvements to make, but for a tiny temporary room it sounds really really good
