You want some kind of map to give you a gradual falloff at the border of the lip, otherwise it will be razor-sharp and terribly obtrusive.  Here's the map that I use:

http://sites.google.com/site/fleshforge/Home/free-stuff-1/v4-lip-mask-bitmap

Making bump fall off that way is something that never occurred to me but is a great idea, I'm gonna do that.

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