I don't have the game yet. I am playing as a Assassin and would like to know the ratio or how many points I need to put in my magika to use the high end Illusion spells.

silent casting is on top of the tree
so think 6 or so but 100 points in illusion

Well, if you're a good assassin you shouldn't need to use too much Health or Stamina when killing so I would do 2 levels to Magicka, 1 to Stamina, 1 to Health. Modify this accordingly depending on how much Magicka you need when casting some higher-end spells or if you need more health.

Just remember you can use gear/apparel to increase your magicka, as well as pots, and also other gear to reduce the amount required for casting illusion spells. You can also use perks to decrease the amount of magicka required. Just don't make the mistake of finding the most power consuming spell you want to cast and then thinking you need to raise your magicka to that level. If you're only looking to really cast illusion, it's pretty easy to gear up for it and still get your stabby on