I believe it actually is 50% and I think 50% is fine.
However, I do like this poll option: "Count up the number of attacks and only let 1/2 of them hit". If there are an odd number of attacks (6 creatures, 1 weapon) then the odd one could then be forced into a random 50%. This would make really limit the "streakiness" of the shield, and there would be certain guarantees.
For instance, if you had 1 photon, 3 dragons, and no weapon, you would be guaranteed at least 1 dragon hitting.
23 creatures and 1 weapon = 12 guaranteed hits.