Static Spark Gap question

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Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net> 

Hi All,

I'm building a 15kv 120ma coil system and want to start with a 
multi-segmented spark gap.  I know that Dr R doesn't like total spark gaps 
in excess of 240 mils but I do plan on setting my safety gap appropriately 
and leaving it alone.  I'm planning on a 16 pipe string with each copper 
pipe being 3 inch long by 0.625 diameter.  Three gap spacing arrangements 
are being considered that will allow me to vary the total gap in 5 mil 
increments.  This will be done by having the center gaps be of smaller 
spacing than what the string is based on.  The smaller gaps will be shorted 
out (as necessary) in various combinations to get the 5 mil increment.  The 
following shows the smaller gaps in the center of the string based on 20, 
25, and 30 mil spacings:

a. 20 mil spacing   .... 20 10 15 20 ....            maximum total gap 285 mils
b. 25 mil spacing   .... 25 10 15 20 25 ....       maximum total gap 345 mils
c. 30 mil spacing   .... 30 15 20 25 30 ....       maximum total gap 420 mils

Questions:

1. For this power level, what is the current thinking on maximum number of 
segments?

2. Is there a rule of thumb on the maximum gap between any two segments?

3. What is the "expected" optimum total gap required for this power source 
and how much would this need to be increased for my 5000 foot elevation?

Answers to these question will help me identify a design center and choose 
an appropriate spacing range.

Many thanks for any help,

Gerry R
Ft Collins, CO