What a league is worth: Premier League to Super League Greece is 0.78 in logs
Domestic ratings cannot be compared across leagues, because a side is only ever measured against its own division. European ties are where leagues meet, and they identify one number per league.
The identification problem
If two leagues never play each other, no amount of domestic data can rank them: every side's rating is relative to its own division's average. European competition is the only bridge, so the whole ladder rests on a few hundred matches a season between clubs that are, by construction, not representative of their leagues.
Which is why the intervals matter
Super League Greece carries an interval of ×0.37 to ×0.57. Any cross-league price built on it inherits that width, and the platform prints it rather than hiding it behind a point estimate.
One goal-rate offset per league, fitted by maximum likelihood on European results with the domestic strengths held fixed and the reference league pinned at 1.00.
Sides carry the league they play in today, and the clubs that qualify for Europe are the strongest in their division, which biases the strong leagues down rather than up.