A good umpire isn’t perfect. They’re consistent, which is harder.

Perfection is not available to anyone making fast judgements from one position. Consistency is, and it is harder, because it means the same standard for the team you like and the team you do not, in the last minute as well as the first. Players can adapt to a strike zone that is slightly wrong, not to one that changes.

Pick a standard you can hold and hold it when it is inconvenient, which is the only part that counts. In an argument between friends, apply the same rule to both sides. Being predictable is undervalued, and it is most of what people mean by fair.