Coffee exists over two continuums, Strength & Extraction.
Strength varies from weak to strong. Typical coffee strengths are about 1.3% TDS to 2.0% TDS, and espresso is typically about 7% TDS to 11% TDS. That means that the brew method oftentimes dictates the usable strength ranges. Strength is usually a preference, as people manipulate brew recipes to cater to their preference.
Extraction varies from under to over extracted. Ideal coffee extraction happens in the range of 18-22% extraction yield. This reflects the nature of how coffee extracts over time. Too little contact time leaves coffee under extracted, which tastes sour and salty. Too long contact time leaves coffee over extracted, which tasted bitter and astringent. Unlike strength, extraction should not be a preference, and we should always be in our 18 to 22% range of values.
If you brewed 40 grams of espresso from a 20 gram dry dose of coffee, and your strength, or TDS% was 10% TDS, that means you’ve extracted 4 grams of coffee solids into the 40 grams of espresso solution. The extraction yield is represented by extracted coffee solids divided by your dry coffee in grams. In this example, 4 grams of coffee solids divided by 20 grams of dry coffee equals 20% extraction yield. This means that the coffee is in the ideal range and likely tastes balanced.
Let’s say you were tasting a coffee over multiple grind sizes. Chances are the shots of espresso you tasted on coarse grind settings, with relatively fast times tasted sour and salty. Chances are that the shots of espresso you tasted on the finest grind settings, with relatively long times tasted bitter and astringent. This is your taste buds interpreting under and over extracted. With a little bit of practice, you should be able to train your tongue to the usable range of 18-22% extraction yield.