devtrackers.gg
It sounds like there was a miscommunication as to your question.
Matching tabletop, you have a ~34% chance to get either a rare or a mythic Retro Artifact.
Again matching tabletop, within that 34%, your chances of getting a mythic instead of a rare are 20%.
Apologies that this information wasn't in the drop rates page. That was an oversight which we will get corrected.
