Those are my estimations based on the data provided earlier. The mW value you get for a certain impedance (35 Ohm for the data provided) is determined by the voltage swing, 1.2 V or 2.4 V for the balanced out, and the impedance itself. Those are the numbers you need to know to estimate (approximately) the amplifier output power for a certain impedance load. But the output power won't give you the whole picture.
You also have to know the maximum output current the amplifier is capable of producing, to determine if it's at least able to drive certain headphones to a certain SPL (ability to achieve certain loudness level on some headphones does not mean the amplifier's ability to drive the headphones adequately and in full). For example, you have two headphone sets both with 40 Ohm impedance (H1, H2), which require about 2 mW input power for achieving 90 sound pressure level (loudness), and an amplifier with 1.2 V voltage swing (and 100 mW output power for 40 Ohm load) produced with 40 Ohm load. Notwithstanding the fact that the amplifier has 100 mW output power (which is 50 times higher than the 2 mW) and H1 and H2 headphones both require only 2 mW to achieve 90 SPL, the amplifier may not be able to drive one of the headphones to 90 SPL. Why? That's right, the headphones the amplifier is incapable to drive are more current hungry, thus they need less voltage but more current while the other ones need less current and more voltage for 90 SPL. As I understand, the limited output current is in major cases the cause of amplifier's incapability to drive certain headphones, because the voltage swing values provided by DAP amplifiers are enough for the most headphones.
As I judge from my experience with my Hifiman HE 500 and ibasso DX 90, there is a very low if any probability of L3 driving Fostex X00 even to 90 SPL via either out (even the more powerful balanced one). And again the total loudness level achieved does not mean the adequate control of the headphones being driven.