TBH, the mechanic still doesn't really work. I just formed Yuan and will lose about one mandate a month due to lacking in tributaries. Yes I could have worn down Ming even further and formed Yuan later, but the goal for the run was to form Yuan and waiting until the very late game isn't really that much fun because I want to use the Yuan ideas.
So you expand east and south. And because I border powerful nations there, I presume that's the reason for losing Mandate.
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I don't really get the calculation. Is it the dev of the countries bordering you? Is it the plain number of tributaries (I have 22)? Am I supposed to make my ally Ottomans a tributary and also every nation bordering me?
Development of the non-tributary countries that border you. Even the vassal of one of your tributaries penalizes you (Mongolia or Yarkand for example). You can realese some countries so you don't border the Ottomans.