Answering my own question for that anybody can find this solution:
FPSText.SetPosition(window.ConvertCoords(20, 20, defaultCamera));
And the reason behind it is:
////////////////////////////////////////////////////////////
/// \brief Convert a point from target coordinates to view coordinates
///
/// Initially, a unit of the 2D world matches a pixel of the
/// render target. But if you define a custom view, this
/// assertion is not true anymore, ie. a point located at
/// (10, 50) in your render target (for example a window) may
/// map to the point (150, 75) in your 2D world -- for example
/// if the view is translated by (140, 25).
///
/// For render windows, this function is typically used to find
/// which point (or object) is located below the mouse cursor.
///
/// This version uses a custom view for calculations, see the other
/// overload of the function to use the current view of the render
/// target.
///
/// \param x X coordinate of the point to convert, relative to the render target
/// \param y Y coordinate of the point to convert, relative to the render target
/// \param view The view to use for converting the point
///
/// \return The converted point, in "world" units
///
////////////////////////////////////////////////////////////
Vector2f ConvertCoords(unsigned int x, unsigned int y, const View& view) const;