SETUP / Options / Display / Hor. Meter: Post-FX dbu
shows the output voltage in dBu.
(0 dBu = -2.2 dBV)

Now you need to know you‘re ‘phones voltage sensitivity, like 102 dB SPL / V for the Sennheiser HD-800.
This is 100 dB SPL @ 0dBu (@ 1 kHz).
(I can calculate the voltage sensitivity for your specific ’phones).

Now you can read the Peak SPL of your ‘phone, but only at the specified frequency, typically 1 kHz.
e.g.: -10 dBu = 90 dB SPL Peak.

Unfortunately hearing loss is caused by the the long term avarage SPL, not so much from the momentary peak, and heaphones are non-linaer across their frequency band.
So finally this all doesn’t help much.

To stay away from hearing hearing loss causing levels:
Listen so that the singer in the song does not sounds much louder than typical human talking.

AUDEZE LCD-5 specs:
Sensitivity: 90 dB/mW (= 0.001 W for 90 dBSPL)
Impedance: 14 Ohm

Voltage for 90 dB: sqrt (14 Ohm x 0.001 W) = 0.118 V

0.118 V in dBu:
20 x (log (0.118 V / 0.775 V)) = –16.3 dBu (for 90 dB SPL) (ear healthy level).

Voltage in dBu for 100 dB SPL:
-16.3 dBu + 100 dBSPL – 90 dBSPL = –6,3 dBu (for 100 dBSPL) (too loud for long term listening).

REMARK:
• dBu reference voltage = 1 mW into 600 Ohm = ca. 0.775 V (exact: sqrt 0.6 V)
• V = Volt
• W = Watt
• dBSPL = deciBel Sound Pressure Level

• 100 dBSPL Peak very loosely equals about 85 dBSPL Average with typical contemporary music.

80 dBSPL Average is the highest safe ear-healthy-long-term listening level.
But - headphones listening has more upper frequency content than typical environmental sounds.

So staying another 5 dB clear of the 80 dB SPL Average margin would be a good idea, -> 90 dBSPL is a good Peak listening level.

With contemporary music:
SETUP /Options / Display / Hor. Meter: Post-FX dbu
would read about -16 dBu on the ADI-2 meters.

With jazz or classic you can go 5-10 dB higher.