You can use the following link for example:
Calculate the power needed to drive your headphone based on sensitivity (db/mw or db/V), impedance (ohms) and loudness (dbSPL)
www.headphonesty.com
I plugged in Amirs measured 40 ohm. He had 318mv for 94dB so I kept changing the dB/V value until it registered around 318mv being used (well 350mv is as accurate as that website would go with it) - which gives a headphone sensitivity of 104dB/V:
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Amir uses an 8dB boost in the bass, so let's say you need a -8dB negative preamp to cover that which effectively reduces the sensitivity of the headphones by 8dB, which means the headphone goes from 104dB/V to 96dB/V. So then we plug in 96dB/V into the calculator and see what we need to reach 94dB desired loudness:
View attachment 246187
So you need around 0.8V and 16mw to drive the Sundara to 94dB when using a -8dB Negative Preamp. This is based on the dB at 425Hz though (which is the point Amir measures at for sensitivity), but for the Sundara that's very close to the same value at 1kHz, which is the point that headphones are normally measured for sensitivity in manufacturer specs, so in this case there'd be pretty much the same.