> Slide rule are good for only a couple of digits of precision. That's why shopkeepers used abacuses not slide rules.
Shopkeepers did integer math, not decimal. They had no need for a slide rule, an abacus is faster at integer math, a slide rule is used for dealing with real numbers.
Yes ... Isn't that my point? I meant it as an example of how neither "basic mental maths and ... sanity checks" nor is a calculator with a 0.5% error rate are appropriate.
I mean that it's not a difference of accuracy, it's a difference of domain. A calculator & a slide rule have the same domain, an abacus and Mayan Quipu have the same domain. The calculator & abacus are faster than the slide rule & Quipu. Similarly a typewriter has the same accuracy as handwriting a document (worse if a pencil is used since typewriter's can't easily correct mistakes), but typing took over book writing because it was faster, not because it was more accurate.
Shopkeepers did integer math, not decimal. They had no need for a slide rule, an abacus is faster at integer math, a slide rule is used for dealing with real numbers.