I am looking for an extension that automatically adds <!--more--> tag automatically after a specific number of words.

The Evermore and others plug-ins I have tried, add <!--more--> just after a paragraph.

I know, a way to do that is manually separate the paragraphs but I am thinking of something automatic. The reason is that I want to have a specific number of preview lines.

Let me know if a plug-in like that exists, or Evermore can be tweaked.