Quote Originally Posted by quindraco View Post
Copper is harder, because I have no idea if or how the slowing breath should impact offensive CR.

Defensive CR: HP 22 is CR 1/8, adjusts to 1 + 1/8 for AC, adjusts to 2 + 1/8 for saves.
Offensive CR: Breath weapon (acid) is DPR 12 at save DC 11. Bite is DPR 7 at +4 to hit. That's average DPR (7*2+12)/3 = 26/3 = 8 2/3. Rounded down to 8, that's DPR 8, or base CR 1/2.
Adjustment based on hit bonus: 1/2.
Adjustment based on save DC: -1/2, so 0.

The average of 2+1/8 and 0 is 1. The average of 2+1/8 and 1/2 is 1. Since the monster is listed as CR 1, I don't know how you got 1.5.
I very much appreciate you spending the time to work through these @quindraco (why isn't there a formal way to reference peeps in giantitp?). In your calculations, you used a breath weapon DPR of 12 when it is actually 18.

Here's what my code produces:

Code:
CopperDragonWyrmling

Inputs:
 - iCR:   1 (initial estimate, affects computed attack bonus)
 - HP :  22 (4d8 + 4; avg 18 + 4HD*+1 (Con 13))
 - AC :  16 (dex=1,natural=15)
 - Att:   4 (+2 prof bonus from CR 1, +2 for Str 15)
 - DPR:  17
    - Att1: acid breath (18) against 2.0 targets once per 3.0 rounds
    - Att2: bite (7) against 1.0 targets once per round
    - DPR = 18*2*0.333 + 7*1*0.667 = 12.0 + 5.0 = 17.0
    - dmg is more from effects with saving throws (12) than attacks (5)
 - RIV
    - immunity to acid

Adjustments:
 - resistances/immunites: 1 immunity
    - no impact on CR
 - saves: proficient in 4 (dex,con,wis,cha)
    - increases effective AC by 2

Outputs:
 - dCR: 2.125 (0.125 + 2) = 2
    - effective HP=22 suggests AC 13 and CR 0.125
    - adjust dCR by 2 ((effective AC - expected AC)/2 = (18-13)/2)
 - oCR: 1 (2 + -1) = 1
    - dmg is more from effects with saving throws than attacks
      (using Save DC rather than Attack Bonus in DMG-274 table)
    - effective DPR=17 suggests DC 13 and CR 2
    - adjust oCR by -1 ((effective. dc - expected dc)/2 = (11-13)/2)
 - fCR: 2 ((2 + 1)/2 = 1.5)

Summary:
  /-----+------+------+-------+-----+----+-----+---------------------------\
  | Var | Act. | Eff. |   Def | Off |  Δ | Res | Notes                     |
  |-----+------+------+-------+-----+----+-----+---------------------------|
  | hp  |   22 |   22 |    22 |     |    |     |                           |
  | ac  |   16 |   18 |    13 |     |  5 |     | 22hp → AC13 (Δ=18-13=5)   |
  |-----+------+------+-------+-----+----+-----+---------------------------|
  | dpr |   17 |   17 |       |  17 |    |     |                           |
  | att |    4 |    4 |       |     |    |     |                           |
  | dc  |   11 |   11 |       |  13 | -2 |     | 17dpr → DC13 (Δ=11-13=-2) |
  |-----+------+------+-------+-----+----+-----+---------------------------|
  | dCR |      |      | 0.125 |     |  2 | 2.0 |                           |
  | oCR |      |      |       |   2 | -1 | 1.0 |                           |
  | CR  |    1 |      |       |     |    | 1.5 |                           |
  \-----+------+------+-------+-----+----+-----+---------------------------/

The example on DMG-275 makes it clear they are rounding average CRs to the nearest integer (2.5 becomes 3 in the example), so an average CR of 1.5 should become 2 (which does NOT match the CR 1 reported in the MM). That said, these situations where a CR is midway between integers is obviously ambiguous ... a CR of 1.5 can probably reasonably be interpreted as either 1 or 2.