So I found a really cool page on the Megasquirt site ( they sell hobbyist EFI systems ) which let's you calculate the breakdown voltage needed to jump the gap of a spark plug in the combustion chamber.
They list a whole bunch of calculations and assumptions that go into their breakdown voltage calculator, which I can't copy to here, but the calculator takes four inputs: plug gap in inches, intake pressure in kPa, the compression ratio and the intake air temperature.
So, the compression on the European version of the 2L Mazda FS engine used in the 626 is 9.7, I'm using 1.1mm gapped plugs which is 0.043 inches, and at idle my engine draws 18.5 inches of mercury which comes out to about 62.65 kPa. My ignition system is wasted spark, so it has to fire two plugs at once, one on the compression stroke and one on the exhaust stroke. The voltage needed to fire the exhaust stroke plug is about 2kV according to what I've read.
So at idle on a warm day, say 35*C air intake temp the breakdown voltage will be about 14kV plus 2kV for the waste spark which gives about 16kv total.
However, those are the easiest conditions. If I look at wide open throttle ( atmospheric pressure of 101.c kPa ) on a below freezing day of say -5*C then the number jumps way up to 22kV + 2kV giving about 24kV.
Now these are the absolute minimum needed to initiate breakdown and ionization of the AF mixture across the gap so current can start flowing to ignite the mixture, but what about things like quenching?
So to account for the extra energy needed to avoid quenching I'm magically going to pick a number out of the air and say let's add 25% to that minimum number of 24kV and arrive at 30kV, which means if I see any spark at all on my adjustable gap spark tester when it's set at 30kV then I'm guessing I'm probably going to be OK.