How to diagnose the need for room treatment?

The ceiling.

Hi Ghstaudio, I assume you are using the Radio Shack microphone/Spl meter (although you did say a good microphone so its probably not Radio Shack). But If you are buying a microphone and you need a SPL meter, just buy the Radio Shack unit. It is more than adequate and the freq correction is done by the REW set up. Also many/most use this combination, myself included, and help in setup is freely available.

Point it straight ahead. It mut be on a tripod at where your head is.

When you get your Freq response graph, waterfall and RT60, please post them here for interpertation. That will be interesting.
Bob

If you are trying to assess room acoustics based on system throughput, aiming the mic forward, or at any speaker, biases the results to the performance of that speaker. Pointing it upwards makes for the least biased results. Also, averaging more than one mic position helps.

Kal

Make sure you have a measurement microphone - an omnidirectional condenser type. For example the Behringer ECM-8000. An ordinary microphone is not suitable

I will first use the RS microphone. I was going to try an AKG CK-32 cartridge (good omnidirectional mike). Finally, I have a rane RA-27 with a measuring microphone that I could try (requires me to build a small interface).

Most likely, I'll never get past the RS.