In .NET Framework 4.6.2 the FormattedText() is Obsoleted, There is an build error.

When I try to build the WPF project with .net framework 4.6.2, I got an error, Because the FormattedText() is Obsoleted as below: 

[Obsolete("Use the PixelsPerDip override", false)]    

   public FormattedText(string textToFormat, CultureInfo culture, FlowDirection flowDirection, Typeface typeface, double emSize, Brush foreground);

The new override method is 

public FormattedText(string textToFormat, CultureInfo culture, FlowDirection flowDirection, Typeface typeface, double emSize, Brush foreground,double pixelsPerDip);

Q: How can I determine the pixelsPerDip ?

Q: How can I use old constructor without pixelsPerDip?, because the pixelsPerDip is useless for my project.