Dex has nothing to do with Magic level.
If your skill goes over 100% it means that your 

IQ

 isnt high enough to raise the skill any further.

IQ X 1.3 = max attainable magic skill level.
Stick a point into IQ and after your next sucessful magic attack you'll get the level.

Cinder.

Does the same work for dex and weapon skills?

i.e. 1.3*dex=max ranged, p/s, and s/s levels

The "Character Guide" locked post in this forum has information about this.
For ordinary methods of building weapon skills, the maximum skill level is (Dex * 1.5).  For training them at the fort it's less.