There are a few different ways to set the ball gap spacing of your arc gaps, depending on whether you know your tower’s base voltage or not.
If you do not know the exact tower base voltage for your station, the easiest method to calibrate your ball gap is to experimentally find the arc threshold for your tower and then back off the setting a bit.
To do this, start by operating at full power with tone modulation and adjust the arc gap tighter and tighter until arcing begins to occur. Then open the gap back up by 1/8” or a bit more (more if the arc gap is outdoors).
This can be done with the large ball gap on a base insulator as well as with individual arc gaps at the ATU output or other places in your system. Doing so will help ensure that your arc gaps are protecting the system as much as possible.