I will contend that you will have MUCH faster games by calling a higher zone! I have proved this time and time again with young umpires I mentor, and prove it to more experienced partners when we work multiple games together. Once I get them away from that "low" strike, and calling anything below the elbow, their games start going much faster, and they are more consistent.

But, call what you like.

The proper head height is your "chin" at the top of the catchers head by the way.

Being too low always has the same effect on the low zone as being too high. There is an optimal height you want to be in. Get too low, people seem to not call enough strikes down low. Get too high, people seem to call too many strikes down low. It is weird. Get your chin at the top of the catcher head and track the ball all the way to the glove. I am betting that you are NOT tracking the ball all the way to the glove, because people that do call excellent games, no matter what experience you have.

Stay focused and tune everything out once the pitcher starts his windup. Your only job at that point is to follow the ball to the glove then rule on it. (yes, I know there are other things to be aware of, but for a inexperienced umpire they just need to focus on the pitch and get that discipline down!)

A new harness is cheap. If you need a bigger one, get a bigger one and adjust the mask to fit properly. Come on, you are getting money to work these games, and you are an independent contractor. YOU provide your tools to work. INVEST! Crap, I have over $1k invested into uniforms and gear!