Hi edwina, 20% vinegar is a safe amount. But don't forget lemon is also acidic, so add vinegar bit at a time.
Too much vinegar is not a good taste.
Ph strips are cheap for testing the acid levels.
Bring to boil and let it high simmer for 20 minutes, that will kill all nasties.
Pour hot into hot steralised jars , put lid on and turn upside down for 5 mins then turn over
Hope this helps