Use _AVG_CHAR_PER_TOKEN in mode=prefix

src/chonkie/refinery/overlap.py

@@ -137,7 +137,7 @@ def _prefix_overlap_token_exact(self, chunk: Chunk) -> Optional[Context]:
             return None
 
         # Take 6x context_size characters to ensure enough tokens
-        char_window = min(len(chunk.text), self.context_size * 6)
+        char_window = min(len(chunk.text), int(self.context_size * self._AVG_CHAR_PER_TOKEN))
         text_portion = chunk.text[-char_window:]
 
         # Get exact token boundaries