from my experience, you tend to fail casts more often when you have more wounds...
If you have high hp gear, the game will see it as your wounds will be at big and nasty longer, since it goes by overall % of hp.
Less hp + more ac = lower % of failure, more effective use of mana overall.
I think you may be a little off base here. Also if you master your spells it will be very rare for you to fail them even on the verge of death (assuming what you say is true)
