Here is a discussion on the debate we had last night. According to 3.5 rules, we were all doing it wrong.
According to the actual rules, one of two things happens when you polymorph (and this is what the argument is about in that post). One side is saying that there is no hit point change at all, so turning into a Wyvern if you have 10 hp means you are now a 10hp Wyvern. The 2nd group says that when you turn into a Wyvern you gain the Wyvern's Str., Con, and Dex bonuses, meaning if you have a 10 con as a wizard and you turn into a 20 con Wyvern, you would gain 40hp as an 8th level Wizard. (+5 hp/level times 8 levels)
I would submit another possible argument based on our general house rules, and the difficulty of using a %-based formula as suggested last night. A possibility is when you turn into a Wyvern, whether you are a 4th Gnome with 30hp or a 38th Level Fighter with 500 hp, a Wyvern always has 100hp.
From that if we go back to Dark_Elf's and Raistca's argument (that it should be based on a %), here is what happens. Say you have 3 characters, one with 10hp, one with 100hp and one with 1000 hp, and as a Wyvern you take 99 points of damage, when everyone turns back into a human, the 10hp char has .1 hp, rounded down to 0 and he goes unconscious. The 100hp character transforms back with 1hp, and the 1000hp transforms back with 10hp, for a total of 990 damage. To me, an avatar level player with 1000 hp should be able to handle damage better than the Wyvern, which in turn handles the damage better than the 10 hp character. Working Polymorph based on a percentage makes that unworkable and a huge boon to characters with less HP's than the target creature and worthless to characters with more HP's than the target creature.
I think the best solution is to use the 2nd defintion up top. If the Sorcerer turns into a Wyvern, he uses his own base HP's and adds the Wyvern's physical characteristics: Str, Con, Dex, Per. Thus, there would always be a benefit to turning into a creature with a higher Constitution, with the drawback that if a player takes too much damage in that form, he is going to be in danger when he changes back. For example, if the Sorcerer had 200 hp's but only a 10 con and changed into a Wyvern, he would have a 20 con, taking his hp from 200 to 200+(his level * 5), in this case 100 (assuming he is 20th level). Now, if he takes over 200hp damage in his new form, he will be in danger when he turns back into a human with only 10 con.
Please leave any comments or things you think we should consider. I think this solution allows Polymorph to continue to be a benefit without overpowering it.