I usually go 1.2 x the minor toroid dia for the height to the base of the
toroid to the last turn on the sec winding.
most pri strike problems arise from overcoupling of the pri-sec coils. Use
an emitter off the toroid to direct the spark to a ground stand. Elevate
emitter (sharp point) by approx 35 degrees with the horizontal. The sparks
shoot out and upward slightly, then curve down to strike your ground stand
(a second sharp point or series of points).
If a classic type coil, use javatc to set coupling in the range of 0.1 to
0.14 and you will not have problems. I like to keep it loose, around 0.12
and seem to have
very good luck with this value.
Dr. Resonance