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Patch 2.3.0 (2007-11-13): Now gains additional benefit from spell damage bonuses. Base absorb value of rank 4 has been reduced (875 from 920).
(wowwiki)
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On current 2.3 live server, my Shadow Ward, with base absorb of 875, and with 1220 +shadow damage, absorbed 1270 Shadow damage. This means that Shadow Ward after 2.3 receives about 32% of your +Shadow damage as extra absorb.
(wowhead)