oh, and the bonus will multiply what you have as a current percentage before you level up. Therefore, 99% before lvl-up is actually the best thing you can have. However, do not "overforce" 99%, once you reach it-be sure to hold it there. I remember a time when I got no bonus. I can only hasard a guess that although it said 99% on my char-page, the database actually contained something close to 99.99 magic skill and considered it 100%. I was kind of upset.:)
What I can account for with numerical data though: I started constructing with Mia at lvl 36. I maintained lvl+4 magic skill on each levelup, with a 66-67% extra on skill. I did 1/3 of the level on TA armies, going to 99% skill. Then I built 2/3 of each level, and on levelup, I dropped back to +3 magic, but from 99% I got to 166-167%, therefore back to +4. I kept building till about lvl 42-43.