http://qt-project.org/doc/qt-5/qfont.html#setLetterSpacing

void QFont::setLetterSpacing(SpacingType type, qreal spacing)

The units for the 'spacing' parameter are not documented at all. Most probably it is pixels, since the parameter is converted to 'int' in the qfont.cpp. At the moment there's only said that "The spacing between the letters can be made smaller as well as larger."

Make the documentation clearer in this respect and add the units to 'spacing' parameter to avoid confusion.