I find MS Excel works great for that kind of stuff. I've used it many times to scale up/down recipes and convert units for recipes with multiple units (tablespoons, ounces, pounds, etc...) and still maintain a very clean look. The trouble lies in making sure you get the formulas right. Computers are not intelligent, just fast. If you tell it to do the wrong thing, it will do the the wrong thing very quickly and not ask any questions.
I would suspect though that Sodastream has made it very easy in that there are only 2 ingredients so it should be a simple matter of setting up a proportion and cross multiplying. Just be careful that your units are the same or you could end up with a mess. What do the instructions say for mixing up a standard batch? I could walk you through the steps.
The other issue would be: is the Sodastream set up to handle that large a quantity? I don't have one so I cannot say; I'll leave that one to you (or someone else on here).