Krepaj: We've tested magic absorption ingame to verify, and Whiskeyjack is correct. Absorption is all or nothing, and it is possible for one character type to have 100% for 60 secs: the Breton. According to posters in that thread mentioned earlier, if you have over 50% passive spell absorption and then use the dragonskin power, you will be completely invulnerable to all magical effects of any kind for 60 seconds. It is a really, really potent ability for a properly perked and equipped character.
Edit: FWIW, some of the in-game descriptions in skyrim are misleading/wrong, and spell absorption is just one of them. For example, the Shield Wall perks in the Blocking skill tree each add 10% per rank to your blocking, despite what the description text reads. That was discovered through testing too. If you take the time to read through the helper guide in the front of each thread incarnation, you might learn a lot about how this game really works.