Math question: Where is zero gain on the Windows 10 level slider?

Hello all. I'm using a Benchmark ADC1 USB to rip vinyl. It uses the generic Windows 10 "USB microphone" driver. I want to set this to zero gain per advice from Benchmark. Unfortunately the levels setting in Windows 10 is calibrated in percent, not dB.

It seems like we should be able to figure out the exact setting for 0dB gain in the Windows 10 sound control panel. I discovered that the "0" level in the Windows sound control panel for the "USB microphone" corresponds to -96dB and the "100" level corresponds to +30dB. However, being a sound level setting, I'm guessing it's logarithmic and not linear. (If it were linear, 0dB would be 76%, which seems too high.)

Does anybody know the math well enough to derive the 0dB setting? I'd really like Windows to just leave the signal completely alone, and use the Benchmark to set the level.