R&N works similar, her special says: “Steals 50% mana from the target and distributes it to allies.”
She steals 50% of the target’s current mana as mana points and distributes them evenly to all of her allies that are alive. If there are less allies alive, each ally will get a bigger chunk of the stolen mana points. If only R&N is alive, she will get all of the stolen mana.
Shacklebolt should work the exact same.
The mana bar on defense is larger than the one on offense, so fully charged targets on the defense team give more mana.
The stolen mana gets rounded down to whole numbers, and again after it gets divided for the even distribution.
Tests for mana stealing: here.