Slide rule are good for only a couple of digits of precision. That's why shopkeepers used abacuses not slide rules.
I have a hard time understanding your hypothetical. What does it mean to hallucinate at the 0.5% level? That repeating the same question has a 0.5% chance of giving the wrong answer but otherwise it's precise? In that case you can repeat the calculation a few times to get high certainty. Or that even if you repeat the same calculation 100 times and choose the most frequent response then there's still a 0.5% chance of it being the wrong one?
Or that values can be consistently off by within 0.5% (like you might get from linear interpolation)? In that case you are a bit better than a slide rule for estimating, but not accurate enough for accounting purposes, to name one.
Does this hypothetical calculator handle just plus, minus, multiply, and divide? Or everything that a TI 84 can handle? Or everything that WolframAlpha can handle?
If you had a slide rule and knew how to use it, when would you pay $40/month for that calculator service?
https://en.wiktionary.org/wiki/couple - "(informal) a small number"
FWIW, "Maximum accuracy for standard linear slide rules is about three decimal significant digits" - https://en.wikipedia.org/wiki/Slide_rule
While yes, "Astronomical work also required precise computations, and, in 19th-century Germany, a steel slide rule about two meters long was used at one observatory. It had a microscope attached, giving it accuracy to six decimal places" (same Wikipedia page), remember that this thread is about calculating devices one might carry in one's pocket, have on one's self, or otherwise be able to "grab".
(There's a scene in a pre-WWII SF story where the astrogators on a large interstellar FTL spacecraft use a multi-meter long slide rule with a microscope to read the vernier scale. I can't remember the story.)
My experience is that I can easily get two digits, but while I'm close to the full three digits, I rarely achieve it, so I wouldn't say you get three decimal digits from a slide rule of the sort I thought was relevant.
I'm a novice at slide rules, so to double-check I consulted archive.org and found "The slide rule: a practical manual" at https://archive.org/details/sliderulepractic00pickrich/page/...
> With the ordinary slide rule, the accuracy obtainable will largely depend upon the precision of the scale spacings, the length of the rule, the speed of working, and the aptitude of the operator. With the lower scales it is generally assumed that the readings are accurate to within 0.5 per cent. ; but with a smooth-working slide the practised user can work to within 0.25 per cent
That's between 2 and 3 digits. You wouldn't do your bookkeeping with it.
New Merriam-Webster dictionary, 1989, def. 4 "an indefinite small number" - https://archive.org/details/newmerriamwebste00spri/page/180/...
Pocket Oxford English dictionary, 2005, def. 3 "(informal) an indefinite small number" - https://archive.org/details/pocketoxfordengl0000unse_p5e4/pa...
The Random House college dictionary, 1975, def. 6, "a couple of, (Informal) a small number of, a few"
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.
Obviously if one product hallucinated and one doesn't it's a no brainer (cough Intel FPUs). But in a world where the only calculators were available hallucinated at the 0.5% level you'd probably have one in your pocket still.
And obviously if the calculator hallucinated at the 90% of the time for a task which could otherwise be automated you'd just use that approach.