The shield does not reduce the damage you take from magic attacks, it increases your magic defense by a percent. This is a stat you can see directly on your character screen.
With the DSS+5 your magic defense increases by 30%. So if your magic def is 100 without the shield it will be 130 with it. Or if you have 200 magic def then with the shield you would have 260.
The damage you take from a magic attack is then reduced based on your magic def, so how much damage is reduced with the shield depends on how much magic def you have without it.
So the numbers in the test above are only relevant for a character with those exact stats and gear setup.
I haven't tested it myself, but I think the magic attack reduction is off as well. I remember the numbers more closely resembling the magic def numbers. But it is possible that I am simply not remembering it correctly.
