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Ok, I get it now. I need to just simply set the SPL Cal to 75 (only during the loopback) so that 75 is 75 during a loopback measurement check. I now get a flat line on top of SoundCard when doing a measure during loopback.

One strange thing though is I must of run into a small bug. Because after I understood this I did and the first time it was now 3db off. So I calibrated SoundCard again. Then it jumped to like 15db offset. Strange. The only thing I did was close software, open again and then it was right on. So something wasn't quite set to what it was showing.

Also I have recalibrate SPL when I rehook the real Mic up (comparing to a handheld SPL) and it doesn't have to be exactly 75db (that's where I was confused). Also have to apply my Mic Cal file at that time (probably before I do SPL cal).