I don't have a package of Sav-a-Chick here either, but I've used it before and I like it. The directions on the package tell you to offer a batch of S-A-Chick + water alongside a normal batch of water. The problem is with ducklings is that I don't know that the ducklings would know to go between the two types of water as needed. Adults, I believe, would. But ducklings, totally different story.
So I suppose I would make up a batch, I believe it is one pkt mixed into 1 gal water, I would then store that inside of a clean container and refrigerate the extra. I'd make a non spill waterer like the cottage cheese container style, and so I'd offer the ducklings that for 3-4hrs, then switch it out with plain water, and so forth. And I would do that for 3-5 days. I don't think I've ever used S-A-C longer than 5 days.
Enjoy your eggs! They are Delish.