Actually I was wrong about Qudelix, it is not that convenient there, because the app does not have a calculator. It only has the mV level on the right of the headroom regulator. For example you get 1e+2 there, 1e+2 is 100, so you get 100mV, means 0.1VRMS. https://www.headphonesty.com/headphone-power-calculator/ - you can calculate the else here.

I was wrong because FiiO devices really have this calculator built into the app and it's connected to the volume level, so you enter sensitivity and impedance of your headphones and get output power and DBFS in return. It's the most convenient way.