When to move on to the next unit prefix, e.g. 900 kB vs 0.9 MB

Is there any established standard or research about what the cut-off point should be for moving to the next unit prefix e.g. moving from "kilo" to "mega"?

I assume it may depend on the unit itself, so I'm specifically asking about bytes (or bits), for example when displaying download progress / remaining.

One approach I've seen is moving as soon as it goes above 1000 (or 1024), e.g. 1000 kbit becomes 1 Mbit. However, it's not uncommon to cut off before that: I've seen e.g. camera manuals say "0.8 MB" rather than "800 KB" for the size of a photo.