What version of D@W are you referencing? The latest version states the following:
Battle ratings calculated using this formula will not line up perfectly with the BRs presented in the Exotic Creatures Roster. The battle ratings in the Roster were calculated by running the creatures’ Domains at War: Battles characteristics through a formula that weighed formation, cleave factor, and other factors. When using the formula listed here, be sure to “benchmark” the outcomes from the formula against similar creatures found in the Exotic Creatures Roster. You can also check out D@W: Battles rulebook for the full details.
Then in D@W Battles, the rules for calculating BR are noted:
If you intend to use your D@W: Battles units in a campaign, it can be helpful to know the units’ battle ratings (BR). In D@W: Campaigns we offered a simplified formula based just on a creature’s AC, HD, and special abilities. Here we share a much more robust formula. Calculating BRs using this formula is not a process intended to be undertaken during play, and is best done using an appropriately-programmed Excel spreadsheet. (In the future, if the popularity of D@W merits, Autarch may release an online tool to assist in calculating unit characteristics and battle ratings.)
To calculate BR, you need to know the following for the unit:
• Armor Class
• Unit Hit Points
• Movement Rate - Walking and Charging
• Formation Type
• Number of Melee Attacks when Charging
• Number of Missile Attacks
• Missile Range
• Unit Morale
With that information in hand, the formula for calculating a unit’s battle rating is:
BR = (Unit AC) x (uhp/8) x (Morale Modifier) x (Speed Modifier) x (Formation Modifier) x (Combat Modifier) / 6
The modifiers are all derived from the unit’s D@W: Battles characteristics, as noted below.
Morale Modifier: Add 7 to Unit Morale, and divide the sum by 7. Take the square root. E.g. [(Morale+7)/7]1/2
Speed Modifier: For FM, IM, LM, or FLY units, speed modifier is equal to (Charging Movement / 9). For other units, speed modifier is equal to (Charging Movement / 7.5)0.33.
Formation Modifier: Use the value listed below the appropriate column on the table below.
FF FLY FM IF IM LM LM
1.15 2.0 1 0.675 0.62 1 1.3
Combat Modifier: Combat modifier is a combination of the unit’s Melee and Missile Values. Melee Value is simply equal to the maximum damage the unit can deal during a charge, including any bonus attacks.
Missile Value is more complex. For LF, LM, and FLY units, Missile Value is:
(number of missile attacks) x (walking movement + missile range) / 2
For other types of units, Missile Value is:
(number of missile attacks) x [(walking movement/3) + missile range] / 2
Units equipped with arbalests and crossbows calculate Missile Value as if they have a walking movement of 0. Units that carry just one spear and no other missile weapons have a Missile Value of 0.
Once Missile Value and Melee Value are known, Combat Modifier can be calculated.
• If Melee Value equals or exceeds Missile Value, Combat Modifier is (Melee Value + 0.2 x Missile Value).
• If Missile Value exceeds Melee Value, Combat Modifier is (Missile Value x 0.05 x Melee Value).
Once you have calculated all of the modifiers, enter the values into the formula. Round the final total to the nearest 0.5. If the unit consists of guard or hunter creatures rather than sentient troops, divide the BR by ½.
EXAMPLE: Let’s calculate the BR of a unit of 120 Heavy Infantry (AC 6, 6 uhp, Movement Rate 1/2/3, Formation FF, 2 Spear & Shield 11+ or 2 Thrown Spear 11+ with Range 2, ML 0). The formula is (Unit AC) x (uhp/8) x (Morale Modifier) x (Speed Modifier) x (Formation Modifier) x (Combat Modifier) / 6. We calculate:
• Armor Class is 6.
• Uhp/8 is (6/8), or 0.75.
• Morale Modifier is [(0+7)/7]1/2, or 1.
• Speed Modifier is (3 / 7.5)0.33, or 0.74.
• Formation Modifier is 1.15.
• Melee Value is equal to the maximum damage the unit can deal in a charge. For this spear-equipped heavy infantry unit with 2 attacks, this is 3, reflecting two successful attacks plus one bonus point from charging with a spear.
• As a FF unit, Missile Value is equal to (number of missile attacks) x [(walking movement/3) + missile range] / 2. For this unit, that is (2) x [1/3 + 2] /2, or 2.33. However, since the Heavy Infantry just carry spears and no other missile weapons, Missile Value is set to 0.
• Since Melee Value of 3 is greater than Missile Value of 0, Combat Modifier is [3 + (0.2 x 0)], or 3.
• Therefore the unit’s BR is (6) * (0.75) * (0.74) * (1.15) * 3) / 6 = 1.91, which rounds to 2.