Mid-letters seems reasonable, but some umps stop at the beltline, or so it seems that way. There is probably also a personal preference for corners and kneecaps, and the front edge of home plate, especially with drop balls and curve balls, i.e., change ups. My issue is pitches fully in the river or, mercy sake, in the opposite batter's box. Not as pitches to entice a swing, but called strikes. Makes batters chase bad pitches as they try to figure out what "consistency" is. For me, if the ball "grazes" the "zone," it was a strike. But, I was a hitter's ump.