How is the Font DPI Scale stored?

I am a developer for the McCLIM <> graphics
toolkit, and I would like it to be able to integrate better with KDE.

Currently, McCLIm has a hardcoded font scale which is problematic when on a
high DPI screen. It's possible for a user to customise it but recomputing
the default fonts, but that's not ideal.

What I'd like to do is to read the KDE "Force Fonts DPI" setting, and use
it to automatically compute the new font sizes. However, it's unclear to me
how this value is propagated to applications. There must be a standard for
this, since a lot of applications is able to pick it up, but I haven't been
able to figure it out.

What is the proper way to read this value?