i dont know the math but, for my 125 storm wizard, 100% critical is 815 and that number raises as you lvl up. im not sure if that number to get 100% critical chance is different for every school. for my other 125 wizards 600 is around 55-60% chance. hope that info helps a bit
What I've noticed across the breadth of my wizards, it seems that your critical rating is also affected by your school.
As you know, the higher level you are, the more your critical decreases (Critical Decay).
Hence, you'll have to take steps to increase this as you level; Jewels, Pets, Gear etc.
While as you mentioned, 's critical rating of 815 is enough for a 100%, I got my to 769 and it's almost the same. So, it appears that the critical rating isn't just based on stats and level, but your school as well.
Crit chance does not increase in linear increments. I don't know the exact words to describe it, but 100 crit chance does not always = +x% crit chance.