Anubis said:
Dragon Size : CR
Tiny Dragons : HD - 2
Small Dragons : HD - 1
Medium-Size Dragons : HD
Large Dragons : HD + 1
Huge Dragons : HD + 2
Gargantuan Dragon : HD + 3
Colossal Dragon : HD + 4
Colossal+ Dragon : HD + 5
So a young black dragon (HD 10d12+20, Medium-size) is CR 10 under your system? It's listed as CR 4 in the MM, and the 'traditional' hack puts it at CR 6.
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Let's compare the young black dragon to some CR 10s in the MM:
Fire giant: 142 hit points, AC 21, +20/+15/+10 (2d8+15), reach
YBD: 85 hit points, AC 19, +12 (1d8+2), +7/+7 (1d6+1), +7/+7 (1d4+1)
AC.....Giant's damage / dragon's damage
10.....3.3
15.....3.9
20.....5.2
25.....7.9
30...12.2
The two creatures have near-identical mental stats. The dragon has special abilities, but none are effective at range except its breath weapon (out to 60 feet) -- the giant is effective to greater distances with its rocks, which deal much more damage (72 if all hit, compared to 15 if the Reflex save is failed vs. the breath weapon). The giant has more feats.
The only real advantage the dragon has is flight.
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The CR 9 greater air elemental , perhaps, is the better comparison because of its own flight.
GAE: 178 hit points, AC 26, +23/+18/+13 (2d8+7)
YBD: 85 hit points, AC 19, +12 (1d8+2), +7/+7 (1d6+1), +7/+7 (1d4+1)
AC.....GAE damage / dragon's damage
10.....2.2
15.....2.8
20.....3.9
25.....6.7
30...12.2
The dragon has 4 more Int than the elemental. The dragon has some special abilities, but none are as good as the air elemental's DR and immunities (as well as reach). The dragon's breath weapon is an advantage, but 15/7.5/0 damage (depending on the Reflex save and evasion) make it a nonissue.