Sodium benzoate the correct amount
Well yeah, adding a preservative is ... just that. 
I'd just stick with the "homemade" part; there is so much misinformation about preservatives that get so quickly spread without research.
You can use it to rinse/sterilize containers without mixing it in but not as effective.
Recipes I've used are ¼ teaspoon per litre (mix with 1 tsp hot water to dissolve). Also can be used to sterilize jars by making a hot water rinse (use similar ratio).
Read Wikipedia on Sodium benzoate if anyone is worried about it as all the hype has been debunked and it's safer than the "natural" potassium sorbate used.