About?! The 14th decimal place of latitude is worth ~0.15mm!

Although that might have been tongue-in-cheek, the word "about" could refer to the accuracy of the position, not its precision.

Precision != accuracy: https://en.wikipedia.org/wiki/Accuracy_and_precision

But the number of digits should reflect the precision.

When pointing at a map, the thickness of your digits should reflect precision.

How accurate are your digits?

Yes. If it's only 5 digits, it should be cut-off to reflect that. Accuracy needs an error estimate.

It's hard if you don't know the precision needed to get to the position you want, easier to just copy/paste and be done with it. How many knows how far 0.001 deg is at a specific location on the globe?

I find it hard to estimate the position from the top-down view compared to when I was there, and I just copy-pasted the coordinates from Google Maps

The smallest possible resolution of GPS in the horizontal is 10mm (6-7 digits) with local differential corrections.

In general, 4-5 digits is the precision to expect.

Hm, then I think my calculation that the 14th digit is worth 0.15mm must be wildly incorrect.

For a 40km earth circumference, I worked out 10^-14 * 360 * 4*10^7 = 0.00014 metres.

I think you want to divide by 360 instead of multiplying by 360 - when you divide by 360, you should get the linear distance represented by one degree of latitude or longitude.