Yes, that's a reasonable procedure. Some points might need attention: When holding the Zerostat, does it charge your body in the process of generating ions? Maybe you need to wear a wrist strap. The Zerostat has two points, presumably one emits positive ions while the other emits negative ions. Does this direction reverse when the handle is released? A static locator needs to be zeroed while pointing at a neutral field, such as towards the middle of the room. Take note if its reading is affected by the Zerostat gun.
The breakdown voltage of your spark gap will be roughly constant if the temperature and humidity are controlled. Over time it will change as the copper oxidizes or spalls off during sparks, attracts soot (carbon) or other contaminants from the air. So you may want to recalibrate after every hundred or so sparks or after a month or so in storage.