I’m not sure what to take away from your first paragraph. Yes, you can’t get an even number by subtracting an even number from another. But I didn’t write two even numbers. I wrote an even one, and an even and odd one. At that point, we’re two levels of abstraction away from the theme of the article (applying rough percentages to things in your life), and into a different area entirely (number theory and arithmetic).
But your sentence about being a technical writer was revealing about this discussion. As I’m sure you know, this is not technical writing. It’s far from technical writing. Had I been doing technical writing, I would have double-checked the calculation.
I would challenge your having applied the expectations of technical writing to an article that is most definitely not technical writing. Is any piece on Medium technical writing? Who is proof-reading and checking? Most technical writing where the same person who writes it also fact-checks it would probably not go far. You need different parties with fresh eyes engaged in each part of the editing process to ensure that the errors are caught and corrected. As you can guess, no one but me (the writer) is proofreading this. So I fall victim to the trap of my same old eyes looking at it again and again. And once more, I’m not doing technical writing here — I would submit that <.001% of folks here are — if any.
Also, had the calculations been of the kind where a miscalculation would result in systemic and bad consequences, they would have been double-checked. But these calculations are not critical. As you note, once you began trying to implement these calculations, you would have seen that there was an error. At that point, the overarching 10% number of that section would still hold. Because I gave a percentage — rather than just laying out the number of hours — I give the reader the opportunity to apply it themselves — thus seeing what my errors may have been.