According to Radio shack, you can buy an SPL meter with integrated mic or a USB mic where a calibration file exists. But I assume a standard calibration file for a mic brand is not really honest, as calibration would have to be done for each individual mic. Do you agree?
The top of the line in Germany is from NTI, but I actually hope not to spend that much and still get something decent (+/- 1 db precision).

However, I could borough this set for a few days from friend. Maybe it helps for calibration?
http://www.thomann.de/de/nti_audio_exel ... _m4260.htm
Then there is another mic from NTI, which would be more within my price range! But I would need an additional SPL meter and I wouldn't want to spend a lot more than that.
http://www.thomann.de/de/nti_mini_spl.h ... wgodh0ILHw
As I find it attractive to be able to do some measurements without REW, I tend more an SPL meter with some other functions, like this one (Phonic PAA3):
http://www.thomann.de/de/phonic_paa3_pe ... istant.htm
But I am not sure, whether the mic is good enough for usage with REW. It can however send the signal to PC via USB.
What do you guys use and what would you recommend? I will be using this equipment only for my own studio build, which I hope to finally become decent! But it's not going to be perfect, as it is to small for a stellar studio. So, my thought was about +/- 1 db of accuracy, as I aim towards +/- 5 dB linearity in the mixing room. Does that sound good to you? How much would you spend on that? Any other tips for equipment?