Has anyone played CK2? the Autocalc is not perfect (When is it?). But I like it and is close to reality. Also there is room for random events. A bigger army is not always the victor. For example;
The following factors affect auto calc in ck2:
1. Battle phases: you have different battle phases (Ranged, shock, melee) (I believe for bannerlord another phase should be added, namely a short skirmish phase right before melee or shock phase) only units with throw weapons engage in this phase)
2. Combat Role:you have different roles to the battle phases namely: you are either attacking or deffing. (this changes non stop in the game and can be affected by overal strategy and who was the original attacking army. This army will always be forced to attack if both armies picked Defensive strategy).
3. Unit Types:There are different unit types (archer, light inf, heavy inf, spearman, light cav, heavy cav, Horsearcher, special units (like elephants)
4. Unit type stats:all unit types have different stats for the battle phases, these stats are also different based on the role they have at that moment. The stats are also changed based on who they are fighting against.
5. Flanks:there are multiple flanks (Left, middle, right)
6. Flank effectiveness: the effectiveness of each flank is partly based on the competence of the leader/ general/ sergeant. its unit type make up and offcourse the flank its fighting against.
7.Overall combat Effectiveness: overall effectiveness is partly based on the main general, which can also compensate for any terrain or weather effects or worsen these effects). This is greatly intertwined with the armies starting morale. Armies that start at low morale just start off badly from the bat. Reducing overall chances to win... So big armies with low starting morale can cause a army 2-3x smaller to win.
8. Damage allocation:each unit deals dmg in a particular phase (archers deal dmg in all phases, but not the same dmg), so more units is more dmg. the dmg is allocated to a specific flank. if that flank has no units to take dmg the dmg is added to other flanks. So for example having a strong right flank and the other has no right flank will cause your right flank to also deal dmg to the enemy mid flank, while the enemy mid flank is only dealing dmg to your mid flank. leaving your right flank free of dmg to assist the flank next to it. (and perhaps extremely increase the chances of a bigger army to win)
9. Weather effects: weather effects. (night reduces vision, snow reduces movement speed, rain reduces accuracy and heavy infantry effectiveness etc)
10. Terrain effects: terrain effects (one of the party get a specific bonus or debuf. for example crossing river is negative. holding high ground is always preferable etc.), woodland heavily reduces ranged and shock phase effectiveness) combined with weather effects, this can be extremely devastating. for example fighting during a rainy day in a forest will be awesome for an army consisting of light infantry with skirmish capabilities) while being devastating for ranged units.
11. Morale: morale (Very random and mutating factor, this changes during the course of the battle, in favor of bigger armies, adds to the randomness of a auto-calc) it is also a type of damage, which doesnt affect the HP of the units but its morale HP. Shock phases are especially effective in reducing enemy morale. and heavy cav shock phase are particalarly effective against ranged units. If moral HP reduces to 0 the unit flees. there is a chance the unit survives the retreat, however if all the flanks retreat and enemy with high cav units will engage with shock phase devastating all units with no morale.
12. Strategy focus: the main strategy (aggressive, balanced, conservative) as the names indicate, decides what strategy you choose, it decides which phases are more likely to happen during combat. Both sides pick a specific strategy focus and a random dice based on unit weight decides what happenes. usually the bigger army dictates what happends. Going aggressive is a full on charge from the start. reducing the ranged phases trying to get mainly in melee or shock phases. Balanced is a nice mix, depending on the situation. Conservative, is mainly holding ground and def. Especially great against cav armies if you got spearman unit types (they deal 100% dmg against horses) any horse in shock phases against a def army with spears is total butchery.
13. Overall strategy effectiveness: The effectiveness of the overall strategy is further enhanched or nerfed by the traits of the main general and its commanders. Great Def generals are great well.. at conservative strategies, giving def boosts to soldiers.
12.(There are probably other factors coming into play, not mentioned by me)
For example during a normal battle, all your flanks engage. your range units start of the battle. so it usually starts with a ranged phase. in this phase ranged units dish out damage against its opposing flank. The duration of a phase can vary, so if somehow an overwelming archer army stay for prolongued time in ranged phase, against an melee army. They will completly devastate them.
Archers will randomly hit units in the opposing flank. however in the Ranged phase only ranged units can attack. Meaning that it doesn't matter if a cav t6 is present or not. if 50 archers are engaging 20 melee cavalry in the ranged phase (the melee cavalry will deal 0 dmg while take, lets say 30 dmg per arrow during range phase each archer shoots 2 arrows = 2x30x50= 3000 dmg... the combined hp of the horses (lets say an soldier is 100hp a cav is 130) 20x130 = 2600 hp. meaning that the full cav dies and another 400 dmg is dissed out to other flanks. The cav flank was not even able to enter melee and shock phase.. This will completely affect the following melee or shock phase, as the opposing side lost its horses before they could dish out damage. However fighting in woodland while raining, while night, it will decrease the amount arrows per archer to be shot during ranged phase, while also further reducing the ranged phase duration because of the woodland type, and because its night and because its raining. causing for example archers to be able to shoot 0.3 arrows per archer instead of the 2 arrows in perfect conditions. The night and rain do not affect melee phases while the night phase gives a boost to cavalry shock phases, causing the archers to be hard hit if not obliterated.