TC spark gap

["Alex Madsen" <alexmadsen-at-netzero-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

I am making my first TC. I am planing on using the "Terrygap"(
http://users.better-dot-org/tfritz/terrygap.jpg) for the coil. My power supply is a
15/60 nst. If my calculations are right 60*.004 in =.24 in. right now I have a
spark length if about 6/16 of an inch (.375). for a Jacob s ladder. 

1.What size xformer was the gap designed for because I am getting a spark with
out any caps that is longer than the total gap length of all 60 pipes combined?

2 .Also can I mount it on ¼ in glass or will the heat cause it to crack? And is
the brake down voltage to low? 

3.How does one calculate the max voltage of an AC xfomer? Do you just
double the
listed voltage.

Thank You 

Alex Madsen