Actually, you want the line from the servo screw to the pushrod connection to make a right angle with the pushrod, if you wish the pushrod to be moved equally in both directions. The connection at "a" does that.

If you need differential movement, placing the servo arm so it is at an angle to the pushrod (as with the configuration as at "b" ) when the servo is centered will give a shorter pushrod movement in one direction. In the picture, for setup "b", when the servo rotates it's arm counterclockwise, the pushrod is moved far less in a straight line than when the servo moves it's arm clockwise. The straight line movement of the pushrod is what the aileron/elevator surface sees. And in the "b" setup, it sees shorter movement when the servo pulls the rod than when the servo pushes the rod toward the aileron/elevator.

Almost always, we want our pushrods to move equally when pushing as when pulling. And to do that, the angle should be 90 degrees at both the servo and the other end of the pushrod. Either end, if it's not 90 degrees, the surface won't move equally.