Just lowered the K to ~.18 according to JavaTC9.1. I also arranged to run the coil on my 240V line so i dont have to worry about the 15A 120V breaker ;-). Solid 6 feet at about 80% input (out of 280VAC) and only requiring 150uS burst lengths. At full 85% input i got some 80" strikes! I didnt dare push to 100% yet.
Steve,
Did you check the results using the original coupling too now that you switched to 280 volts input? Just switching to 280 volts can increase the spark length in some systems for a given variac setting.