Squealing feedback noise when setting levels

I have a few questions / problems related to setting levels. The first is that I was taking some measurements last night and while adjusting the mic input levels up I would get a squealing feedback noise that you hear when a mic is too close to the speakers. The problem is that if I don't push the mic input up the levels are too low. If I set the mic input just below the level that feedback occurs, I can still hear a bit of feedback at the end of the measurement tone. Is this corrupting the measurement? What am I doing wrong?

My second question is that I really don't understand how to set the levels and calibrate the SPL reading. If I set the volume on my prepro so that the pink noise generated by REW is at 75 dB on my RS SPL meter, the level check always comes back too low. This is why I was increasing the mic input level to help get the level in the green. Increasing the mic input up doesn't get the levels all the way in the green before feedback sets in though, so I have to increase the volume on the prepro to about 78 dB. Do I enter 78 dB into the SPL calibration field? Am I doing this right?

I also noticed that the mic is live the entire time I'm using REW (if I speak into it I can hear my voice through my sound system). Is this normal?

Thanks in advance for any help you can provide.

- Tim