Same for Windos, but heres the question macOS oriented !
I want to get this information:
com.apple.print.preset.Scaling100
That is (in the example above) the Scale Setting, set by default at 100%.
The second information is about the paper format (page): A4 or A5 or as set by the user.
The idea is to be able to print a Page (front and rear) at A4 or A5, following user asking.
Actually, in a different project, same client, I print A4/100% to PDF and the client use the pdf for reding AND printing purposes. The scaling and page format is done at the PDF printing time (not me).
But I will have to improve this print technic in a brand new project.
Clues and advices are allowed 
BTW: the used Xojo is 2015r1, and will certainly be current at delivery time.
And yes, X-Platform (Linux / macOS / Windows 7 [now, 10++ then]).
Nota: in another project, I set the default % to 70 and (apparently), the OS print to PDF at the correct size (A4 is used by default). But this project is in house only.
Yes, this is not good (as I saw when using a different % value 