Why grind size matters
Water takes the taste out of coffee only while the two are in contact. Espresso runs for about 25 to 30 seconds, so the grounds are fine. A French press steeps for 4 minutes on a coarser grind. Cold brew sits for 12 to 24 hours on a coarse grind. The shorter the contact, the finer the grind.
Fix the taste
A sour or weak cup usually means the grind is too coarse. Bitter or harsh usually means too fine. Either way, move to the next size on the chart, not to the far end. Then taste. If the cup still leans the same way, move once more.
Why every chart is approximate
Nobody has set an official standard for grind sizes. Each chart is one maker's convention. Charts disagree. For the French press, one gives a narrow range and another a wide one. Grinders differ too, so a friend's setting may not match yours. Start from the chart, then adjust by taste.