How to bevel in proportion? I have been doing this again and again and searched but cant fix it.
I think the bevel is not on the center of the edge

How to make better bevel?
there's a short cut on edit mode: 
1 . select the edges you want to bevel. [works with faces too]

2 . press [ctrl]+[b]. 

3 . dragg the mouse to set the angle of the bevel. 

4 . scroll the mouse weel to segment(loop) the arc.