Similar to Thornmail, this item's passive interacts with itself. Given the lowest amount of MR possible with this item (30+30=60), then the damage taken calculation (100/160 = .625), then the raw damage conversion (250/.625), the shield will absorb around 400 raw magic damage.
Hexdrinker'sshield can help its wielder fight through otherwise lethal magic damage. This may take opponents by surprise, especially if the opponent tries for a risky burst kill but fails to penetrate the shield.
The shield's short duration challenges users to make the most of the moment. Try relying on the shield to see you though enough magic damage to ensure your own kill, or to make a grand escape.
The shield can block a considerable amount in the early and middle stages of the game, but it does not scale and may fall off in the late game should opponents obtain more powerful spells and AP items. For late-game power, try upgrading to Maw of Malmortius.
Increasing magic resistance increases the total damage absorbed by decreasing magic damage taken; some consider this to be "indirect scaling."
Whether purposeful or not, the minimum amount of raw damage that's able to be absorbed by Hexdrinker is the same as the shield amount on Maw of Malmortius.