Every other calculator on the internet answers “how much power do these headphones need”. This one runs it the other way: given the amplifier you already own, how loud can it actually make them — and is that a number you should be worried about in either direction.
Almost certainly not, and the number above shows why. Headphone drivers dissipate milliwatts. An amplifier capable of 120 dB SPL is not a threat to the driver, because you would have to sit at 120 dB to get near its thermal limit, and 120 dB damages your hearing in well under a minute. The fragile component in this system is you.
Headroom is a good thing. A source that reaches 115 dB when you listen at 80 has 35 dB in hand for the peaks in the music, and spends its life loafing. A source that only just reaches 85 dB is working flat out all the time.
This one is false for headphones, and it is worth knowing where it came from. In loudspeakers there is a real mechanism: a clipped waveform has a higher average power than a clean one, the passive crossover routes the extra high-frequency energy to a tweeter rated for a fraction of the system's power, and the tweeter cooks. Rod Elliott's analysis of exactly this puts the failure down to sustained power, not to distortion itself.
Neither half of that transfers. A headphone is a single full-range driver with no crossover to redistribute anything, and an amplifier clipping at any volume you would tolerate is delivering a small fraction of a watt. The real risks are turn-on thumps and DC faults — a completely different failure — and, once again, your ears, because a clipping amplifier is one you have already turned up too far.
One caveat on the number itself: manufacturers' sensitivity figures vary by around ±3 dB between measurement standards, and quoted power ratings are often taken at a voltage the amplifier cannot sustain. Check the voltage row in the table — plenty of “200 mW into 300Ω” claims quietly need 7.7 volts.