Help with using Vorpal Tribble's CR Calculator

[DrKerosene]

I've been toying with Vorpal Tribble's CR Calculator, and I'm increasingly sure that I'm making mistakes. Originally I was looking at an Ogre, Basilisk, Greater Barghest and Dragonkin, since I thought the Dragonkin was probably closer to CR5. I was suprised it didn't quite hit CR4.

CR3 Ogre from the SRD29 divided by 4.5, (4HD): 6.4
Armor Class: 16 (+1)
Special Attacks: None (+0)
Special Qualities: Darkvision and LLV (+1)
Bonus Feats: None (+0)
8.4 divided by 3: 2.8
End Result: Basically CR 3

CR5 Basilisk
45 divided by 5, (6HD): 9
Armor Class: 16 (+1)
Special Attacks: Petrifying Gaze (+1)
Special Qualities: Darkvision and LLV (+1)
Bonus Feats: None (+0)
12 divided by 3: 4
End Result: Actually CR 4

CR5 Greater Barghest67 divided by 5, (9HD): 13.4
Armor Class: 20 (+2)
Special Attacks: SLAs, Feed (+2)
Special Qualities: Change Shape, Pass Without Trace, DR 10/magic, Darkvision, Scent (+3)
Bonus Feats: None (+0)
20.4 divided by 3: 6.8
End Result: Basically CR 7

CR3 Dragonkin from Draconomicon, pg 15038 divided by 5, (7HD): 7.6
Armor Class: 17 (+1)
Special Attacks: Rake (+1)
Special Qualities: Darkvision, Detect Magic (+2)
Bonus Feats: None (+0)
11.6 divided by 3: 3.867
End Result: Almost CR 4Adding any normal armor would probably increase the CR to 4.

I usually bundle LLV with Darkvision for a single +1, or something similar with other minor abilities. However, if I count each as +1, then I usually end up increasing the CR of a creature even higher, like the Greater Barghest. Is it possible I just selected several poor creatures for this exercise? A couple more then, Dire Lion, a Huge Monstrous Spider, and a Large Water Elemental.

CR5 Dire Lion60 divided by 5, (8HD): 12
Armor Class: 15 (+1)
Special Attacks: Improved Grab, Pounce, Rake (+3)
Special Qualities: LLV and Scent (+1)
Bonus Feats: None (+0)
17 divided by 3: 5.666
End Result: Basically CR 5, good

CR5 Huge Monstrous Spider52 divided by 5, (8HD): 10.4
Armor Class: 16 (+1)
Special Attacks: Poison, Web (+2)
Special Qualities: Vermin Traits, Darkvision, Tremosense (+2)
Bonus Feats: No feats at all (+0)
15.4 divided by 3: 5.133
End Result: Also basically CR 5, great

CR5 Large Water Elemental68 divided by 5, (8HD): 13.6
Armor Class: 20 (+2)
Special Attacks: Water Mastery, Drench, Vortex (+3)
Special Qualities: Elemental Traits, Darkvision, DR 5/- (+2)
Bonus Feats: None (+0)
20.6 divided by 3: 6.867
End Result: A little under CR 7.Water Mastery seems more like a penalty to the creature. Drench seems pretty sub-par with magical/alchemical light options, and I don't know how often a Vortex will be used on average, so I guess that evens out the CR to 5.6 or so, but I can't go three creatures without being off by several CR.

Am I over thinking, missing something, or is this normal when trying to calculate CR? I was briefly considering trying to continue the MM2 thread before this.

[sirgarberto]

Wait, it calculates from total HP? Doesn't that end up making high CON a penalty rather than something good? I mean, if you have two... say, plants, one of them has a 20 in CON and the other has an 8, for the same score you could have creature 1 with hp 47 (5d8+25) [vorpal score: 10.44] and creature 2 with hp 59 (17d8-17) [vorpal score: 9.91]. Creature 2 will have +9BAB, same Fort (+6 for HD, -6 for CON difference), +6 Ref, +4Will, more non-bonus feats, higher skills (none of which seem to matter for the calculator), higher HP (because of the calculation changes for higher HD)... and has a lower score. If I gave it a CON of 3 instead, it could even have some epic feats, and still be possible to make what the vorpal calculator would consider a CR5 creature.

[137beth]

Wait, it calculates from total HP? Doesn't that end up making high CON a penalty rather than something good?
No, it calculates from both total HP and HD. More HD means each hit-point is worth less.
Note, when I use it, I avoid counting HP from toughness and similar feats, because those hit-points are factored in in the feats category.

[sirgarberto]

No, it calculates from both total HP and HD. More HD means each hit-point is worth less.
Note, when I use it, I avoid counting HP from toughness and similar feats, because those hit-points are factored in in the feats category.

I know that. That actually makes it even worse, since creatures with a CON penalty will end up with more hp than creatures with high CON for the same CR (note that in my example creature 2 has 12 more hp than creature 1). Not to mention all the benefits of a high HD count, none of which enter the calculation, as I elaborated above.

[atemu1234]

Note the calculator was never designed to be the end-all, be-all test of a being. Simply a rough estimate for a DM.