I’m generally OK with the grab a known element and scale. However, I am concerned about doing this if someone has kerf corrected the file, or even knowing if kerf correction has been applied. For the gift card, if I change the document properties to 72dpi, the hole for the chicago screw become 5.08mm, which looks to be kerf corrected for a 5mm screw. Without knowing this, I would have scaled this to a 5mm hole.
For completeness, in Inkscape you can go to document properties, select “in” as the display unit, and read off the scale in “user units per in”.
So you’re saying the the DPI doesn’t have any effect? That doesn’t appear to follow from what I’ve been reading. For example, there was a huge upset when Inkscape changed from 90 to 96dpi as the default. There are many posts about that, here is one