Over or Under Extracted?
When it comes to grind sizing, what you need to figure out is if your coffee is over-extracted, under-extracted, or just right. This, again, is a pretty complicated issue. However, the “get started” answer is that if it tastes sour, it might be under-extracted, if it tastes bitter, it may be over-extracted. If under-extracted, adjust your grind setting to be finer. If over-extracted, go coarser.
The best way to adjust your settings is a little at a time and test as you go. That way, you can know once you've hit the sweet spot.
Keep in mind though that neither bitter or sour is necessarily “bad” in coffee. In fact, bitter can be pleasant - as seen in dark chocolate and sour can also be pleasant as in citrus fruits. But both bitter and sour need a sweetness to balance them out so what we are looking for is an “out of balance” cup, where it is way too bitter or sour with no sweetness.
When To Readjust Settings
Once you have that perfect setting though, there may be times you need to re-adjust. One of those times would be if you change brewing methods. The general rule there is that the more open the filtration system, the coarser the grind. For example, the metal plunger in a french press requires a coarser grind than a paper filter in a pour-over method.
Another instance is when using different beans. Besides taste, another way to tell if a particular bean needs a different setting is brew time. If it brews faster than usual, go finer, slower than normal, go coarser.
One of the main benefits to adjusting your settings though is that it helps you get to know your grinder. Many people set their settings once and leave it but regular adjustments is a great way to enhance your coffee drinking experience.