The absortion effects from atronach stone and the perks are multiplicative not additive. The damage reduction cap according to UESP is 85%. so getting the magic warding perks are fine.
Actually, the atronach stone/perk IS additive: 50% stone + 30% perk = 80% spell absorb. Magic resist perks and enchants are also additive with each other, not multiplied against each other. Spell absorb has no cap, but magic resist has an 85% cap which stacks with elemental resists (also 85% cap for each of those), but are applied separately.
Formula for magic defense:
Spell absorb is applied first to incoming effect, and will either absorb all of the effect/damage, or none of it -- based on the percent chance
Magic resist is applied next, assuming spell absorb failed. Caps at 85%
Elemental resist is then applied to any remainder of damage that made it through magic resist. Caps at 85%
Note that effects like poison spit and disease are considered "magic" by the game engine, and are susceptible to being absorbed. If you get blasted by a frostbite spider's venom attack, you might just get a recharge of magicka instead of getting hurt.

If you absorb more magicka from an incoming spell/effect than you have room for in your blue bar, the overage is ignored and dissipates harmlessly.
-Loth