Just to check, if I have the following:

Atronach Perk (30% Absorb)

Atronach Stone (50% Absorb)

Agent of Mara (15% MR)

3 Alteration perks (30% MR)

Ring of MR (23% MR)

Amulet of MR 23% MR)

then I should have 80% Spell Absorption, and 85% Magical Resistance, making me only take 3% of any magic damage (shock, frost, fire, etc). Is that correct?

Yep. Magic attacks will feel like a piece of paper hitting your chest.

Yes, Twinkle fingers will have a hard time hurting you.

Absorb% is a chance to absorb all incoming damage/effects or nothing. If a spell goes through, it affects you 100%.

So, you have 80% chance to ignore incoming spells. Those that hits you hits for 15% damage. If you want to reduce that further, you'll have to use the induvidual elemental resistances which reduces damage after magic resistance has done it's part.

Magic = lolz you threw a frostbite at me

I have absorb on my necklace, atronauch stone, and dragonskin once per day