The decision when to harvest the grapes is vitally important. The links to the videos below explain how to assess grape ripeness. It’s not just about sugar levels, but flavour and tannin ripeness too. And don’t forget to look at the colour of the pips!
https://www.facebook.com/latasque/videos/574270872976104/
https://www.facebook.com/latasque/videos/257689888406439/