Usage of global variable for dynamic value that must be declared compile time [closed]

Question

I've been trying to make a work-around for global usage; however, I can't seem to find an alternative.

I'd like to create a C++ suffix function which converts pixels to millimeters. A custom gui library I've been developing separates OS calls and the actual widget system in two separate headers (IoC); therefore, I do not have access to system-level DPI calls when creating widgets (which is when I'd want to use the suffix function). A kludgey work-around I've considered is maintaining a global variable / singleton. For example:

// widgets.h

...

class SystemProperties { // ! << SINGLETON CLASS HERE
  std::pair<double, double> dpi = {1, 1};

  SystemProperties() {}

  static auto self() -> SystemProperties& {
    static SystemProperties system_properties;
    return system_properties;
  }

 public:
  static auto set_dpi(const decltype(dpi)& dpi) {
    SystemProperties::self().dpi = dpi;
  }

  static auto get_dpi_x() { return SystemProperties::self().dpi.first; }
  static auto get_dpi_x() { return SystemProperties::self().dpi.second; }
};

double operator"" _inch_x(long double value) {
  return value * SystemProperties::get_dpi_x(); // ! << GLOBAL USAGE HERE
}

double operator"" _inch_y(long double value) {
  return value * SystemProperties::get_dpi_y(); // ! << GLOBAL USAGE HERE
}

...

// win32_implementation.h

#include "widgets.h"

...

auto init() {
  UINT dpiX, dpiY;
  GetDpiForMonitor(
      ::MonitorFromWindow(::GetDesktopWindow(), MONITOR_DEFAULTTOPRIMARY),
      MDT_EFFECTIVE_DPI, &dpiX, &dpiY);
  SystemProperties::set_dpi({dpiX, dpiY}); // ! << GLOBAL USAGE HERE
}

...

Of course I could implement some kind of additional IoC that does dpi conversion, but that would miss the point of being able to use the suffix operator. Perhaps I am getting too hung up on that function? What are some other work-arounds if possible?