Sodium benzoate the correct amount I would like to know the ratio for using sodium benzoate as a preservative in our jams and relishes. We state on our product that it's home made and no added preservatives are added, would this be wrong when sodium benzoate is added?
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.