Once you have tuned your systems for best performance (under all power
levels), the last adjustment tweak should be for "best" coupling
(i.e., best performance with no racing sparks). Unfortunately, this
adjustment is different for every coil. It is mainly dependent on the
quenching capability of the gap, bang size, resonator size, and
robustness of the secondary insulation system (including field
control between the toroid and primary). At present there are no
magic formulas or tools (other than some rules of thumb, such as keeping
"k" below 0.15 or so).
Optimal coupling for YOUR system must be experimentally determined.