We found this concentrate to be an excellent and relatively cheap compliment to Elderflowers, although we used carton grape juice too. We were warned against using too much carton stuff as it's made from desert grapes.
Suggest you use the big tin in the proportion advised on the label (ie makes 5 litres) but use the specified cold water as an initial infusion for the elderflowers with sugar and yeast, but don't add the grape juice until you've drained-off the flowers.
If you want to pad-out to 10 litres you could use a further 5 litres of supermarket carton grape juice, but i wouldn't advise using much more water, or it could be a bit thin tasting.