How to align text in custom QWidgetAction with text in standard QAction?

[Diracsbracket]

Hi,
I have a menu that contains a standard checkable QAction, and below it two custom QWidgetAction. Each QWidgetAction consists of a QHBoxLayout with a QLabel and a QComboBox as shown in the picture below.

I would like to align the texts of the QLabel with the text of the standard QAction, i.e. along the vertical horizontal line shown, using the left margin property of the layout.

Where can I find the info to determine how wide the check box is, and the various margins in the QAction menu item?