qtvirtualkeyboard Custom Style "scaleHint"

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Question: How Do I Compute the "scaleHint"?

I am developing a custom style for the qtvirtualkeyboard. According to this page, it says:

This value is determined by the physical size and the design size of the keyboard.

I understand that the physical size of the keyboard and the design size of the keyboard are used to compute the scaleHint, but it doesn't say how it uses those to factors to determine it.

Can someone explain what that actually means and also... how do I compute the actual value of the scaleHint?

Thanks in advance.