Just to add to Big Jim's response, the voltage required to jump the gap is not calculated, it is the actual point when the voltage jumps "on its own". Once the spark starts, the voltage cannot rise any further as the spark is "shorting" the voltage. As compression values rise (throttle increase, load increase) the voltage level needed to cross the gap rises. The gap acts as a resistor in the circuit. Resistor wires and resistor plugs actually cause a minimum voltage that will be available to the plugs. The old point sytems actually had the same response, but coils then had less available voltage.
Bigger gaps cause hotter, bigger sparks, AS LONG AS THE COIL CAN DELIVER THEM. If the necessary voltage cannot be delivered, a misfire occurs. So too little a gap requires less voltage-gives a smaller less powerful spark (and a possible misfire). Too large a gap exceeds the capacity of the coil and causes a misfire.
On a further note, sparks always jump easiest from sharp edges. When the electrodes are worn down into a rounded shape, the required voltage increases. Thin wire platinum plugs maintain their edges longer both from the hardness of the platinum and the thin form factor. That's why the dual plat's work best in our engines.
