Standardizing the Polymorph Subschool

[Man-thing]

I'm usually good at spotting patterns but I'm at a loss to generate an algorithm that fits the mess know as the spells of the polymorph subschool. Admittedly, I don't have a lot of spells to go from (PHB II and Dragon Magic) but still.

Spell Level: 3rd - lets a player character change into a Pseudodragon (CR 1, BAB +2)
Spell Level: 4th - lets a player character change into a Troll (CR 5, BAB +4, +30 hp)
Spell Level: 5th - lets a player character change into a Dragonne (CR 9th, BAB +9, +30 hp)
Spell Level: 6th - lets a player character change into a Young Red Dragon (CR 7th, BAB +13, +60 hp)
Spell Level: 7th - lets a player character change into an Aspect of Platinum (CR 12th, BAB +12, +110 hp)
Spell Level: 8th - lets a player character change into an Aspect of Tiamat (CR 13th, BAB +13, +100 hp)
Spell Level: 9th - lets a player character change into a Mature Adult Red Dragon (CR 18th, BAB +25, +150 hp)

There just seems to be no clear progression.

I'm thinking it should be something like this:
SL 3rd: BAB +1 to +2 (range of 2)
SL 4th: BAB +3 to +4 (range of 2)
SL 5th: BAB +5 to +7; +30 hp (range of 3)
SL 6th: BAB +8 to +11; +60 hp (range of 3)
SL 7th: BAB +12 to +15; +90 hp (range of 4)
SL 8th: BAB +16 to +19; +120 hp (range of 4)
SL 9th: BAB +20 to +25; +150 hp (range of 5)

Opinions?

 

[DrunkonDuty]

Good luck finding a pattern.

I use the polymorph re-vamp rules available on Giant in the Playground. They make sense and are fair and balanced. Yay Rich Burlew! (and for OOtS too obviously).