One thing to put in is that while regular arrows will kill more men, many more men will recover from them than fire arrows. Think about it, in rtw, one arrow 'kills' a guy, but in real-life it would depend on where you were hit, and even if you were hit in the torso you might still survive if it was fired from way off. Typically speaking I find that if you have a unit of archers that you accidently allow within range of another archer unit which uses regular arrows on them, even 90/160 kills among your unit will result in full recovery after the battle, provided you have a surgeon. Now fire arrows on the other hand, one shot one kill. Once you've set a man on fire and he's burned all over his body the arrow wound is almost superfluous. Units hit by fire arrows tend to recover few or none of the men lost, even with a surgeon ancillary. So how can we implement this? If you're charging into a city with a full stack of troops but only the second level walls, you need arrow support so you can drive off the enemy from the gate to avoid a bottleneck. If you fear the stupidity of your archer's firing sequences allowing them to launch a volley at your own troops, use regular arrows and if they manage it not nearly as many of your men will actually die.