Ralph addressed this in his writings and in the development of the MPF. In short, he states that 100 points is probably the point at which most players would start to notice a difference. For others it might be 150 points. What you notice might also be influenced by the type of designs you are comparing. My experience is that most will see a difference at 100+. I think a key point is the type of player comparing. If you hit the ball in the center of the face all or most of the time, you will notice variations less from one level to the next. Keeping that thought in the context of the discussion, I do think it is more likely to notice a 100 point difference the lower the number is, like you described, noticing the difference between the 405 and the 508. You should definitely notice a difference between 508 and 711. Features to get an iron to 711 (or above) are those that provide more stability, forgiveness and should provide more consistent and solid feel, all based on the mass and dimensional characteristics of the design. I do believe, and again experience with players tells us, that certain players will definitely see a difference between 700 and 900 MPF.