For offensive infantry fire, am I missing something: it doesn't seem to matter how many factors of infantry are firing, there doesn't seem to be a DRM that reflects 3 stacks of 30 SP each firing together vs one stack of 6 SP firing.
You can fire with as many stacks as you want, but all you get is the benefit of firing with more than 4 SP!
This is one of the "quirks" of this game system. We're all used to "More Good! Less Bad!" when it comes to attack factors. Doesn't matter here for fire combat.
It took a while for me to get used to this and avoid piling on troops at the firing line, or try firing from multiple hexes.
Instead, I learned to only set up the most effective troops to fire (light troops if possible) and save the "big guns" for the melee attack. This is where the large SP totals and multiple hex attacks become advantages.
IN this game system Infantry combat is not too effective in most cases; if you want to punish your opponent with fire combat you need to bring the artillery.