This brief looks at juice extraction, preparation, pasteurisation, bottling, quality control, and equipment suppliers.
I have been passed your enquiry on sodium benzoate levels and have sought advice from the Food Standards Agency, hence the delay in replying.
The maximum level in fruit juices is 150ppm or parts per million.
This means 150g of benzoate per 1000 litres. I am sure you can work it out from there.As your batches are small, say 5 litres I suggest you make a 10% stock solution of benzoate dissolved in water which could then be added using a small measuring cylinder. This would save the expense of an accurate balance.
I do make suggest you check with your local standards people as different countries have different rules.
Hope this helps.