It is the average rating of you top 5 champs excluding your mastery setup. In game there is only one way to find your prestige and that is by removing all of your masteries and averaging the rating of your top 5 champions. Outside of the game there are many prestige bots on the Line app or online, just search for one and it should be pretty easy to find. I don't think I can link you to one on the forums because we aren't allowed to share links from other websites.
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https://mbch.guide/en/kalkulyator-prestizha/