How does the ring of friend protection interact with the shield master ability? If the familiar triggers is are they now taking 1/2 my dmg twice netting a 25/75 split? Or if I trigger the ring do they take 1/2 then o take back 1/2 at a 75/25 split?
I don't immediately see a reason the two skills would not stack, and looking for RAW issues, not specific GM issues.
RAW are sadly silent on the subject of multiple damage-splitting effects at one time, so while the numbers you propose are certainly reasonable you will need to ask your GM.