How to make application scaling high dpi?

Question

C++ / C#

In my MFC application, I'm trying to expand a dialog box to full screen, but it expands to more than full screen. If on my laptop (win7) it is still possible to configure, then on a high dpi monitor, it generally turns out to be an orgy. Unfortunately, I can't test on a high dpi monitor. so I want to know what I'm doing wrong.
There is the following method for scaling, but it seems to be useless.

HDC screen = ::GetDC(0);
giDpi = GetDeviceCaps(screen, LOGPIXELSX);

int dpiScale(const int &iSrc)
{
  return MulDiv(iSrc, giDpi, 96);
}

Actually further, in the dialog box, and init I do the following:

CRect rect;
  GetParent()->GetWindowRect(&rect);
  SetWindowPos(NULL, 0, 0, rect.Width() - dpiScale(100), rect.Height() - dpiScale(100), SWP_NOREDRAW);

I found scaling information based on the monitor ( GetDpiForMonitor() ), but the essence is the same there.