if you know how much sugar is in the other juices - and it will be on the nutrition label - you can work out what you need :
230g of sugar per litre should give around a 12% wine (according to Moley's WOW How To.. )
So if you are doing 5 litres, you need 1150g of sugar in total.
If the ribena is 104g per litre, then 2 litres is 208g. Then add the sugar values from the other juices, subtract that amount from the grand total, and then you know what to add.
Say the grape juice is 140g and the cranberry is 80g, 1 litre of each - that's 240g, add that to the ribena 208g, and you have 448g of sugar in solution already, so add another 702g and you will now have the total 1150g you want :thumb:
Substitute the actual sugar values from the juices you are using, and away you go :drink: