Rather than zest, you can just throw a whole citrus fruit in the food processor and put the results in with the berries - although I've found entire grapefruits can leave a hint of bitterness in the finished sauce. I've had best luck with cara-cara oranges (cross between naval orange and grapefruit).

If you have a kitchen scale, weigh your berries, and the sugar. Once you know the ratio you like, you can replicate with as many bags of berries as you like.

For instance, the ratio I use: per 1g cranberries 0.52g water & 0.66g sugar - but I like it quite tart.