Hey oz,
I usually try to get at least 3 1/2" of tang into a handle no matter the length of the blade or handle. More is better. You'll have to do the cm conversions. ;~)
But I'd rather have the tang at least 3/4 the length of the handle if I can. It's just a personal preference.
Some guys only go a couple inches, but the knife will balance easier and be stronger the more tang you have to work with to start.
With your fire starter idea, you can always notch the tang so that the stick will fit down the center of the tang. You'll be drilling all the way through anyway, so you can make the tang nearly to the end like a T/D or through tang, slot/notch for the fire stick and still have a strong, well balanced knife.