Sigurthr wrote ...

Generally you set your spark gap to fire continuously at slightly less than the rated Vpk voltage of your tank capacitor, otherwise a misfiring will cause too much voltage to build up and you'll destroy your capacitor.

So, with no tank (primary and capacitor) connected set the spark gap to fire continuously, and then widen it to the point it no longer continuously fires. Now close the gap ever so slightly. If you designed/chose your capacitor correctly the gap should be set perfectly now. Hook up the tank and fire up the coil with the primary tapped either at the center or where predicted by modeling. Employ trial and error to determine the best tap location where streamers are longest.

cbfull wrote ...

The spark gap determines how high the voltage can get on the capacitors before they discharge. That's all there is to it, no actual "tuning" occurs at the spark gap.

Think of the spark gap like the height of a glass. The taller the glass, the higher the water level in it has to get before it overflows. The discharge across the spark gap is like this. The capacitor(s) charge up until they overflow, and the spark gap does an excellent job of containing/controlling this overflow of charge so to speak.

This is how I would do it:

Set the gap width to something reasonable. It doesn't matter where you set it at this point but it's better not to make it too wide while you are tuning. A centimeter or less is probably good. Leave it alone for now. Don't change it again until you finish tuning (tapping) the primary.

Now you can do some actual tuning by finding the ideal tapping point in your primary. Just select a tap point, power it up, and stick with the tap point that gives you the best output at your terminal.

Now you can set your spark gap to its widest, which will put the highest voltage across your tuned primary, and thus your best output.

Does this help? I apologize if I tried to explain something you already understand.