This is me calculating FiM Power Levels. Enjoy
So by the story of Season 4, Tirek had escaped from Tartarus, but he had become so weak that it took so long for his magic to recover to begin stealing magic. That points that Tirek must be at some power to steal magic. Once he reaches that point, he has the ability to steal a pony’s magic. The more power he steals, the more powerful he gets. He first begins to steal unicorn magic, then earth and pegasi, and then alicorn and chaos magic.