flutter making if statement outside the class

hello everyone i am creating a flutter app and i need to do a simple if statement to decide the device type i can do it inside the original class like this and it will work fine of course

class _MyHomePageState extends State<MyHomePage> {

  @override

 Widget build(BuildContext context) {

    double font;
    double categoriesFont;
    double navHeight;
    double navIcons;
    if(DeviceInformation(context).width > 600){
      font = fontTablet;
      categoriesFont = categoriesTabletFont;
      navHeight = navTabletHeight;
      navIcons = navTabletIcons;
    }else{
      font = fontPhone;
      categoriesFont = categoriesPhoneFont;
      navHeight = navPhoneHeight;
      navIcons = navPhoneIcons;
    }

    return Scaffold(....

but instead of putting this code in every page i have, i want to make a class that will do this if statement and it will return font , categoriesFont , navHeight and navIcons variables so that i can use them as much as i need in other pages

i want to do something like this

//this class decides device type and use its corresponding percentages
class SetDeviceType{
  final context;
  SetDeviceType(this.context);
  double font;
  double categoriesFont;
  double navHeight;
  double navIcons;
  void deviceType(){
    if(DeviceInformation(context).width > 600){
      font = fontTablet;
      categoriesFont = categoriesTabletFont;
      navHeight = navTabletHeight;
      navIcons = navTabletIcons;
    }else{
      font = fontPhone;
      categoriesFont = categoriesPhoneFont;
      navHeight = navPhoneHeight;
      navIcons = navPhoneIcons;
    }
  }
  double get responsiveFont => font;
  double get responsiveCategoriesFont => categoriesFont;
  double get responsiveNavHeight => navHeight;
  double get responsiveNavIcons => navIcons;

}

and then simply i can do

DeviceType(context).font 

to get the font variable after deciding the device screen type and use it, but i didn't find a way around it.

the main idea here is to make this if statement reusable and to make the code cleaner.

any idea how i can make this happen ? it doesn't have to be a class anything will do the job is welcome i.e function, etc

Thanks in advance.