Make is simpler: 15% chance to drop a shard, done for each time the wither has a difficulty point.
E.g. at 5 difficulty the wither has 15% chance to drop a shard + 15% chance to drop a shard + 15% chance to drop a shard + 15% chance to drop a shard + 15% chance to drop a shard. On average Withers drop 15% of difficulty as shards.