I think TrueRTA is free, or at least many of it's features can be used for free. What you propose may be possible, but I don't know anything about your mic. At a minimum you could use a Radio Shack SPL meter & some test tones (generally downloadable for free is you look around- and there's an online chart that's designed to compensate for the "known deviations" of the RatShack meter). The latter method is actually described in the Audioholics review of the Onix R-DES parametric EQ.