Hi Greg,
That seems to me like a good match. Regarding a rule of thumb,
it would depend I think on whether you're aiming for absolute max spark
length, or maximum numbers of streamers, or some intermediate
number of streamers for a certain "look". I'm sure you could use
a larger toroid for your secondary (depending on the power level).
For example Gary Lau uses
I think a 7" x 25" toroid for his 6" x 27" secondary and it works
very well, but I think he gets mostly just one streamer since he's
aiming for max spark length and uses about 3000 watts I think.
If you begin with your power input level, and calculate the expected
spark length, that should give an idea of a good toroid size figuring
for spark length = 3.5 times toroid diameter. A 56" spark seems to
work well for a 5" x 16" toroid. For a 4" x 16" toroid you might
get more streamers than for a 5" x 16" toroid. By using the
tall secondary, then it's less important to use a sub-toroid
below the main one, to prevent ground or primary strikes.
I'm sure others will have some good comments too....
Regards,
John