Below is a visual depiction of the moving strike zone in slow-pitch softball.
As you can see, the strike zone will vary with the height of the pitch as well as the height of the batter. Where the batter stands in the box has no bearing on the strike zone. ASA rules define the zone as front knee/back shoulder while the batter is standing adjacent to the plate. If a batter is not standing adjacent to the plate, the umpire must make a mental adjustment by taking the size of the batter and visualizing him/her adjacent to the plate.
If everyone were the same size and always stood adjacent to the plate, calling balls and strikes could easily become mechanical. That, however, is not the case.
As seen below, two pitches could land in the exact same spot behind the plate with one being a strike and the other being a ball (known as 'over the top').
If any part of the ball crosses any white portion of the plate, it is a strike. As you can plainly see in the drawing below, a pitch can land in an area wide of the plate (or mat if one is used) and still be a legitimate strike.
The ground has no impact on the strike zone. Batters that point to the ground as a reference point are clueless as to what constitutes the slow-pitch strike zone. Umpires that point to the ground and use that as a reference point for the call are either uninitiated or lazy and are doing nothing more than perpetuating the players' inaccurate belief of what constitutes a ball or a strike. Unfortunately, umpires who call the zone correctly usually take more heat than those who do not.
Umpires that work SBA games are asked to kindly refrain from the practice of ground-pointing.