each level increases your performance by 5%. you do not add up your total percentage from your levels, but rather, 5% of the base, then 5% of that, then 5% of the previous.....
so, 12.5-(12.5*0.05)=11.875 (this is for the first level of your skill)
11.875-(11.875*0.05)=11.28125 (2nd level)
11.28125-(11.28125*0.05)=10.7171875 (3rd level)
10.7171875-(10.7171875*0.05)=10.181328125 (4th level)
10.181328125-(10.181328125*0.05)=9.67226171875 (5th level)
even though this is appearently not the way the game mechanics work, this would be the proper way to calculate your targeting time, not by adding your total % from the skill.