Howdy good sir. I like to recall of myself equally an "unofficial expert" in the field. I take tested this, and come to the conclusion that it is an "all or nil" issue. The Atronach Stone provides a fifty% adventure to either completely absorb a magic effect, or do nothing to it. For an easy example, nosotros can consider a magic attack of 100 burn down damage. The Atronach Stone has a l% chance to absorb ALL of the damage, and a 50% gamble to do nothing. This means sometimes you accept 100 damage, and sometimes you take 0 damage.
The Atronach stone is one of only iii powers that has the absorb magic effect. The others beingness the Atronach perk (100 Alteration) and the Breton sixty second daily ability. Magic Absorption does non have a cap, so you can take this to 100% by using the Atronach rock and being a Breton and activating your daily ability. Y'all can as well exercise information technology every bit any race with a Necromage Vampire permanently. At 100%, you will never receive a disease, never have any harm from whatsoever class of magic, including poisons, dragon jiff, vampire's life bleed, spriggan'due south bug attack, etc.
If you have whatsoever questions feel free to ask!
EDIT: Everything on the UESP wiki is correct. Skyrim:Spell Absorption - UESPWiki