OK, I admit it: my strike zone looks like a stop sign. Closer to the middle it is wider.
In most every situation (save a couple atrocious school ball games I might do) I will negotiate the width of my zone. If I call one between the letter and the armpits, or if I ever call one where the entire ball is not above the knee, I have erred.
Your zone may differ...and is slightly higher in many cases, especially east of the rockies
The strike zone is indeed defined in the rule book, just like the infield fly. But ordinary effort for Player A is NOT the same as ordinary effort for Player B. Could go on ad nauseum, but don't want to beat the dead horse any further...