Help determining ACR for mass combat

[Tom4444]

Quote:

Army Challenge Rating (ACR): This is based on the CR of an individual unit from the army and the army's size, and scales like CRs for monsters. To determine ACR, see Table 4—15: Army Sizes and apply the modifier for the army's size to the CR of an individual unit in the army. If an army is cavalry, use the mount's CR or the rider's CR, whichever is higher. For example, an individual orc warrior 1 is CR 1/3, so an army of 100 orc warriors 1 is ACR 1/3; an army of 500 orc warriors 1 is ACR 3 (4 steps greater than the standard 100-unit army). If a group's ACR would be lower than 1/8, it doesn't count as an army—add more troops until you reach an ACR of 1/8 or higher.

Could someone help me understand the text in bold? I probably missed a simple rule on how this is done but I'm not sure where to look. According to the table a huge sized army has an ACR of CR+4, since the CR of an individual orc is 1/3, shouldn't the adjusted value be 4 and 1/3 before rounding? How was the 3 derived?

[avr]

They're counting one step up from CR 1/3 to 1/2, a second from 1/2 to 1, then two more steps up from 1 to 3. Not how I'd have done it if I were going from 1 orc to 5 orcs, but that's how the writer of the mass combat system rolls.

[Tom4444]

Thanks, I searched based on that interpretation and I found the relevant clause. May as well post it here in case someone runs into the same problem.

Quote:

Adding NPCs: Creatures whose Hit Dice are solely a factor of their class levels and not a feature of their race, such as all of the PC races detailed in Chapter 2, are factored into combats a little differently than normal monsters or monsters with class levels. A creature that possesses class levels, but does not have any racial Hit Dice, is factored in as a creature with a CR equal to its class levels –1. A creature that only possesses non-player class levels (Adept, Aristocrat, Commoner, Expert, or Warrior) is factored in as a creature with a CR equal to its class levels –2. If this reduction would reduce a creature’s CR to below 1, its CR drops one step on the following progression for each step below 1 this reduction would make: 1/2, 1/3, 1/4, 1/6, 1/8.