I'm not sure it's a question of a certain ride quality at a given angle. My interpretation of it is that you want to install the shock so that its linear de-accelleration qualities are more and more progressive as it approaches full compression....without going over-center. At least that's how I install 'em.

Henry Rifle gave a valid explanation on here quite a while ago, saying that if you trapped a piece of string under the left rear tire and swung the other end of the string up to the frame or crossmember from its end at the right rear tire, you would describe the arc that tire would make on bump and that would be the ideal line on which to install the shock.

You never want the angle between the centerline of the shock and the centerline of the axle to exceed 90 degrees at full bump. Ideally, it should be right at 90 degrees at full bump to take full advantage of the shock in my opinion.