Using a RS SPL meter

What is the proper way to use a RS SPL meter in order to measure the room response. What I have done is set the meter to "C" weighting, "fast response". In particular, I wanted to see what was happening in my room from 200 Hz down (my room is 14'5" wide x 19' long with coffered ceiling).
This is the part I'm not sure I did correctly. Using a Sterophile Test CD, I played 200Hz track, and adjusted the volume of my preamp in my room until the RS SPL measured 90 db. I then played each of the following tracks (with lower freqs), and recorded the db reading from the SPL. I put the RS SPL on a tripod, at ear level, from my listening position, and pointed the meter directly forward (aimed between the two speakers).
Wow, the response dropped significantly below 60 Hz? I'm talking about a drop of 18db by the time i got down to 25 Hz. (My speakers are Acoustic Zen Crescendos, which are full range). I understand (from another thread here on A'gon) that the RS SPL is deficient in the lower frequencies, and that I need to add db's to the reading I get (however, even after db's to the response, the response below 50 would still be down over 12 db).
So,
#1 Am I using the meter correctly?
#2 If not, what is the correct way to measure room response using the RS SPL meter?
#3. Any thing else you want, to add or need to know?
In advance, thanks!
Louis