The Spirit Realm

Here is how a 20% focus that degrades at 10% per level over works

lvl 70 = 20%
lvl 71 = 18%
lvl 72 = 16%
lvl 73 = 14%
lvl 74 = 12%
lcl 75 = 10%

Right. The degradation is 10% (5% for spell haste) of the focus amount per level, so that in 10 levels (20 for spell haste) it will have absolutely no effect.

And it's a flat 10% per level off of the initial focus amount, not 10% off of the already degraded amount from the previous degrade.

L65 with no focus, 5/5/5/5/5 secs:
[Tue Mar 06 16:20:19 2007] You begin casting Malos.
[Tue Mar 06 16:20:24 2007] You feel very vulnerable.
[Tue Mar 06 16:20:35 2007] You begin casting Malos.
[Tue Mar 06 16:20:40 2007] You feel very vulnerable.
[Tue Mar 06 16:20:52 2007] You begin casting Malos.
[Tue Mar 06 16:20:57 2007] You feel very vulnerable.
[Tue Mar 06 16:21:09 2007] You begin casting Malos.
[Tue Mar 06 16:21:14 2007] You feel very vulnerable.
[Tue Mar 06 16:21:25 2007] You begin casting Malos.
[Tue Mar 06 16:21:30 2007] You feel very vulnerable.

L65 with L67 focus, 4/3/4/4/4 secs:
[Tue Mar 06 16:21:46 2007] Your Cord of Potential shimmers briefly.
[Tue Mar 06 16:21:46 2007] You begin casting Malos.
[Tue Mar 06 16:21:50 2007] You feel very vulnerable.
[Tue Mar 06 16:22:02 2007] Your Cord of Potential shimmers briefly.
[Tue Mar 06 16:22:02 2007] You begin casting Malos.
[Tue Mar 06 16:22:05 2007] You feel very vulnerable.
[Tue Mar 06 16:22:40 2007] Your Cord of Potential shimmers briefly.
[Tue Mar 06 16:22:40 2007] You begin casting Malos.
[Tue Mar 06 16:22:44 2007] You feel very vulnerable.
[Tue Mar 06 16:22:55 2007] Your Cord of Potential shimmers briefly.
[Tue Mar 06 16:22:55 2007] You begin casting Malos.
[Tue Mar 06 16:22:59 2007] You feel very vulnerable.
[Tue Mar 06 16:23:10 2007] Your Cord of Potential shimmers briefly.
[Tue Mar 06 16:23:10 2007] You begin casting Malos.
[Tue Mar 06 16:23:14 2007] You feel very vulnerable.

L75 with L67 focus, 5/4/5/6/5 secs:
[Tue Mar 06 16:23:20 2007] You begin casting Malis Rk. II.
[Tue Mar 06 16:23:25 2007] Your vulnerability fades.
[Tue Mar 06 16:23:25 2007] You feel very vulnerable.
[Tue Mar 06 16:23:31 2007] Your Cord of Potential shimmers briefly.
[Tue Mar 06 16:23:31 2007] You begin casting Malis Rk. II.
[Tue Mar 06 16:23:35 2007] You feel very vulnerable.
[Tue Mar 06 16:23:39 2007] Your Cord of Potential shimmers briefly.
[Tue Mar 06 16:23:39 2007] You begin casting Malis Rk. II.
[Tue Mar 06 16:23:44 2007] You feel very vulnerable.
[Tue Mar 06 16:23:49 2007] Your Cord of Potential shimmers briefly.
[Tue Mar 06 16:23:49 2007] You begin casting Malis Rk. II.
[Tue Mar 06 16:23:53 2007] You feel very vulnerable.
[Tue Mar 06 16:23:57 2007] Your Cord of Potential shimmers briefly.
[Tue Mar 06 16:23:57 2007] You begin casting Malis Rk. II.
[Tue Mar 06 16:24:02 2007] You feel very vulnerable.

Lastly, a screenshot, with belt equipped (and no other relevant focus):



So... sweet dreams!