Is it correct and safe to assume that "use by" dates are less precise the longer in future they are?
"Use by" dates are pretty imprecise to start with. They're designed to be conservative, and they have a huge amount of leeway.

To the degree that they mean anything at all, I'd say that their precision is proportional to their duration. They might be conservative by, say, a factor of 2: eggs that are good for "1 week" are likely to be good for at least 2, and a box of cake mix good for "6 months" will be good for at least a year. And so on.

The fudge factor may well be closer to 4, or even 10. I've certainly kept a lot of things much, much longer than the expected dates. The only things for which expiration dates even come close is meat and fish, followed by milk. Those are things that "go bad" in obvious and unpleasant ways. Everything else just kinda gradually degrades, rather than suddenly becoming toxic.