Pro 21:1 The king’s heart is in the hand of the LORD, Like the rivers of water; He turns it wherever He wishes.

John Calvin uses Proverbs 21:1 as evidence/illustration that God controls all thoughts of the minds of all people:

With regard to secret movements, what Solomon says of the heart of a king, that it is turned hither and thither, as God sees meet (Prov. 21:1), certainly applies to the whole human race, and has the same force as if he had said, that whatever we conceive in our minds is directed to its end by the secret inspiration of God.
Calvin, John. The John Calvin Collection: 12 Classic Works . Waxkeep Publishing. Kindle Edition.

The context of Proverbs is non-existent. The surrounding verses are unrelated:

Pro 20:30 Blows that hurt cleanse away evil, As do stripes the inner depths of the heart.
Pro 21:2 Every way of a man is right in his own eyes, But the LORD weighs the hearts.

Without context giving definitive meaning to the text, it is hard to see how Calvin uses this as evidence of his position. John Calvin’s reading on this is not the only possible meaning. It could easily be a rule of thumb, or it could be just referring to those events that God wants to make occur through a king.

Typically statements like this need to be read in context of the writer’s perceptions about God. In that way the reader can understand to what extent and purpose the phrase is useful. For example, in a story about a created being, Marduk, the Enuma Elish describes Marduk as determining the courses of the gods:

[Marduk, who is Tutu]
Truly, he is supreme in the Assembly of the gods;
No one among the gods is his equal.
Tutu is Ziukkinna, life of the host of the gods,
Who established for the gods the holy heavens;
Who keeps a hold on their ways, determines their courses;
He shall not be forgotten by the beclouded.

In context, Marduk has risen to the throne through divine combat and can be replaced. The power statements that are attributed to him are not claim that Calvin would make of similar phrases, but just claims of general guidance, power, and ability to manipulate. It is not absolutely, and is limited by textual context.

In the same way, Proverbs does not have to be Calvin’s reading. It could easily be one of those generalizations or broad power claims. There is nothing in context requiring or suggesting Calvin’s reading.