I'd rather not install PowerAuras to get that one effect as I think the new default UI does a good job with everything else. I'd like to find a single use addon for this if it's available.
Yeah, it's called Blood Shield Tracker. Did you even try to google it? It's a solid addon, takes a little configuring. It gives you estimates about how much of a shield you have at a given point in time, in addition to tracking the previous 5 seconds of damage intake and converts to an estimate of the size of the ds heal and corresponding shield you would get at any given moment.
You can use power auras to track your diseases, certain boss mechanics and general stuff that would normally cause you to take your eyes off your character to look at your buff bar. Once you start figuring out uses for it, you use it more and more. The more you focus your attention to the middle of the screen where your character is, the less likely you are to miss something.