I think of it as “outside of my Lightning setup, what will I spend these UTXOs on, “someday” and what’s my best guess for the value then.

If your bull case is that 100K or 1M sats will buy you a car, maybe that’s a good increment. I would keep a range, some smaller and some larger, for various future uses.

It’s hard to know the future, though, and may require multiple iterations.