When you poke around the ASP.NET Ajax javascript files you can find some pretty interesting stuff. This one might come in handy. It's used in a number of the animations. It and the animations can be found in PreviewGlitz.js.

Let's say you have two points, 100 and 30, and want to know the point that is 85% of the way from 100 to 30. Call the interpolate method.

Sys.Preview.UI.Effects.Glitz.interpolate(100, 30, 85);

The first value is the start point, the second the end point, and the latter the percentage. The result in this case is 40.5. At 90% it is 37. At 95% it is 33.5. You get the point.

So if you need it...now you know it's there.