I meant to make it a real library if somebody had said that he would need a display for this in his addon but doesn't had the time to write the whole tracking and calculation crap (and keep it up to date). Since no one has, I save a lot of time :D.
I think every other question raised was already answered by another person in this thread :-).
I'm the author of GridStatusShield which tries to track the amount left of shields on the raid. To do this I wrote a seperate file called LibShieldLeft.lua which throws events when shields appear,disappear and especially when they absorb something.
At the moment it calculates only the exact total size of a shield for PW:Shield, Divine Aegis and Valanyr bubble and only if cast by the player himself (not for any shield cast by another player in the raid). For every other shield it assumes a certain minimum total amount (the base amount for this shield without spellpower or any other bonus).
For GridStatusShield it worked fine since almost a year now.
Since I saw today that the author of Vuhdo used that file for his addon (which is ok for me) I wanted to ask if there is a generell interrest for a library like this (to be used for other unit frames). Then I would make it a real stand alone library which can be used by other addons.