That would be the proper way to do it with that setup, however you are at the mercy of the noise floor of your motherboard's mic-in/line-in. The D10's noise floor could very well be far below than what you're seeing on the graph, and is masked by the quality of your motherboard's ADC capabilities. That's why dudes like Amir shell out for super expensive analyzers; their insanely low noise floor/distortion capabilities are what's needed to get an accurate measurement.