> orders of magnitude work by rounding from .5.

No, orders of magnitude are exponential, not linear, so conventionally “on the order of 1 billion” would be between 100 million × sqrt(10) and 1 billion × sqrt(10), but “billionaire” isn't “net worth on the order of 1 billion” but “net worth of 1 billion or more”, or, when used heirarchically alongside trillionaire ans millionaire “net worth of at least one billion and less than one trillion”.