how could I set DPI Scaling as OS default Or Get OS default scale percent

I try to make a welcome UI as my application first MainUI, application resolution default as 800*600, the UI is very small(I enable high dpi support, and running on 4k). I want set my UI as OS default scale percent.
For now, I find UDPICustomScalingRule can do it, but I don’t where I can find OS scale percent.