Had a clinician tell me that mathematically any pitch that was 6 feet in the air at it's apex and didn't hit the plate should be a strike - given that it is not too high in the zone. Don't know that this is true having never checked the math myself - and I don't really prescribe to this philosophy.
You have said it correctly that many like to call Church mat ball (a mat is placed behind the plate and if the pitch hits the mat, it is a strike.) While others, like myself, call the strike zone based upon the batter's size.
Honestly though, what I have found is that the mat ball calls are what people readily accept; whereas, calling the strike zone based upon where the ball crosses the batter (as if they were standing at the front of the plate) gets some of them excited and gets me some strange looks. The catcher always wants to argue when a flat pitch hits behind the plate and I call a ball. They also want to argue when a big arch hits near the back of the batter's box and I call a ball. When a relatively flat pitch comes across the batter's waist and the catcher makes their catch well behind the batter's box and I call strike... then the batter wants to argue.
My response is usually, "That pitch was flat and too low, or too high and came down behind the batter. The batter couldn't possibly have hit it." or "That pitch came across the plate below your waist, how could it possibly not be strike?" Of course these comments assume the pitch height requirements were met - there is always the illegal pitch call to save your bacon.
If you understand baseball, you will also understand that where the pitch is caught (or hits the ground) has very little to do with where the pitch crossed the plate... but the catcher's ability can sure help sell those "questionable" calls.
Slowpitch has got a big zone... 17 inch wide plate plus the width of the ball (any part of the ball over the plate qualifies as a strike)... makes a zone that is nearly 2 feet wide! And it is a hitter's game so call lots of strikes.
PS. I also recommend covering up the black at the back of the the plate to alleviate any arguments about whether it hit the plate or hit the dirt (Dirt is what you want so you can call a strike.)
Have fun!
