Units of measure — Refused, never guessed (2)

• Deleting a unit that is in use is refused, and the refusal names where it is used, table by field. Deactivate it instead: it leaves the pickers and every value already recorded in it stays readable. The arithmetic: factors and converted values are decimal in the application and numeric(28,12) in the database, never binary floating point — 0.30000000000000004 in front of somebody is not a rounding style, it is a bug. A chained conversion runs at full precision and rounds once, at the point a person reads it. The imperial factors are the defined SI equivalences: an inch is 25.4 mm and a pound is 0.45359237 kg, so metric-to-imperial round trips are exact rather than approximately exact.