I was thinking about draw, and being able to draw straight back shows your stroke is in line and your hitting on the vertical axis and that contact with the ob is dead on. So I was doing this drill today.
With the 1-5 lined up as shown sink the fifteen in the corner and draw the cb back to even with the 2 ball without drawing into the rail or the two ball. Move the cue ball back next to the three ball and draw back to even with the two. Then to the four ball and five always drawing back to the 2 ball.