If you changed your trigger to "When the enemy hits me/or an ally" I would let you have the 20% chance instead.
Because for them to hit, that is the situation that had to have happened in that instant.
Other than that, I can't see timing it right when it is random without using something that gave you insight into their state in that exact moment.
If they flicker in and out on the half second, and it typically takes a person between 1 and 2 seconds to process a change to their environment and react to it, then even the "readied actions happen right before the action that triggered them" means that right before you could see them, they were phased out. Since your trigger is them phasing in, and readied actions happen prior to that occurring, you would actually be arguing that you go from a 50% chance to hit to a 100% chance to miss as they are explicitly ethereal at that moment.