Using a consumer-grade SPL meter to do an in-room response curve is tricky. If you're careful, it will allow you to identify a few gross anomalies, but it just isn't accurate enough (or, as some of these posts suggest, consistent enough) for more than that. Frankly, I think your ears may be a better tool.
Rat Shack SPL meter calaibration
I understand that the Radio Shack SPL meters are not real accurate. I have the $60 digital readout unit. I have been playing around with it to try to work out a hump in the bass from my system using bass warble tones. Can anyone tell me where the problem areas are with these meters and what sort of compensation you need to do to interpret the info it gives you?