Yesterday Jose helped me with this, so let me share it with you all:
GetScaleFactorForMonitor was first introduced in Windows 8.1 and I stopped updating the headers before it. You can use:
DECLARE FUNCTION GetScaleFactorForMonitor IMPORT "Shcore.dll" ALIAS "GetScaleFactorForMonitor" ( _
BYVAL hMon AS DWORD _ ' HMONITOR hMon
, BYREF pScale AS LONG _ ' DEVICE_SCALE_FACTOR *pScale
) AS LONG ' HRESULT
thanks Jose!