When I use the Create Multiple Copies command, the Overall Size values are calculated differently depending on the zoom % of the document I’m working on. Which makes predictability of the width / height area completely impossible.

It’s also has something to do with the resolution of the monitor. On my MacBookPro, I need to setup my Zoom to 66% to get a practical math logic. On my Old Surface Pro 3, it need to be 43.2%.

This juggling of math make this feature really close to useless.

Tell me, what is Acrobat's mathematical logic here? And how can I use this features precisely?

Make that size reading/display the physical size in the document, instead of linking it to % Zoom and/or monitor resolution.