Let's just go with this.
Spark gap is inversely proportional to the density of the gas. Since you're just doing a simple combustion, let's start you off at 1/8".
Also, in my experience, you need a larger, hotter spark to ignite unstoichiometric fuel mixtures. If you're using hairspray, lighter fuel, starter fluid, etc., I would lengthen the spark gap and increase amperage of the ignition source as well.
If you use metered propane, MAPP, or butane, you can probably make the spark smaller than 1/8", though. This is just because the mixture is more reactive, so you need less area and less work from the spark to ignite it.
You said you're a very exact person... so I'm sorry that all I can do is pull numbers and theories out of my ass. But, hey, they work for me.
