Unfortunately, the information you have from Vagabond is a bit off, and you can trace it if you re-read your post.
Things are actually simpler than they sound.

Since Easter is determined "astronimically" (the first sunday after the first full moon after the equinox) you don't need a calendar to determine it. Just need to watch for the moon.

The Bulgarian Orthodox Church uses the Gregorian calendar, unlike some other orthodox churches - e.g. the Russian or Serbian. But Easter is the same for all orthodox, since it doesn't depend on the calendar.