How to generate node title from first few words of long text field?

I want to auto-generate the title of a node from the first few words of the body text ("long text" field type). The token help for the field says "The following properties may be appended to the token: value (Text), format (Text format)". I've tried the format [node:field-some-field:summary] but the title field remains blank. Do I have to manually define the "summary" value of my field somewhere? If I leave the token at the default [node:field-some-field:?] then the title field is populated by the content type's machine name for some reason.