Here's how I like to setup the circular draw shot. The blocking ball is frozen to the rail with its edge even with the side pocket point. The object ball is three ball widths away and a half ball width back.

I align the cue ball with the object ball so they point to about the 3/4 diamond point on the short rail away from the corner pocket. Remove the two spacer balls and place the hanging ball. This shot can be made with two blocker instead of just one, in case one gets too easy for you.

I use a rail bridge, but I let my cue stick rub against the nose of the cushion. Basically, you want to have a level stroke, but you're restricted, so just get it as level as you can. I prefer to aim at 7 o'clock, but it can be made with straight draw. Hit it as low as possible with a quick, hard stroke. Keep the grip loose throughout the stroke. I tend to concentrate on my bicep to give me the power for the stroke.