Was this the one you referred to? While this post is older, it is as good a starting point that you are going to get. The settings as a baseline haven’t changed. Speed of engraving has increased so that adds some more variables.
Soap as a material would be highly variable. That means that the best way to figure it out really is to start testing at high speed and lowest power. Lower LPI too.
As Chef John says, “You after all, are the Story Musgrave of your soap engrave.”