If you are concerned about calories and sugar, I can't help wondering how using only the powder mixed with water, yet using a double powder serving, helps at all. The reason the nutritional information for just powder is given is because that is what the can contains. Slimfast recommends using milk to mix - hence the nutritional info available with milk.
If you mix the powder up as they recommend, then for 257 calories you are getting 14g protein, 6.6g fat and 32g sugar. That's using Optima French Vanilla - the one I use most often. If you mix two servings of powder with water, then for 254 calories (essentially the same) you are getting 10.4g protein, 3.2g fat and 40g sugar. For me, I'd rather have the protein than the extra sugar. If you are worried about saturated fat in 2% milk then use skim or 1% and add some more healthy fats like flax meal or olive oil. I often use lower fat milk and add flax meal.
Unless you can't tolerate milk then I really don't understand what you're saving this way. Not trying to belittle your decision - I just don't see the benefit. If you don't like using milk products except kefir then why not mix the slimfast powder with that? It wouldn't exactly be a drink, but it would save you mixing sugar with the kefir as it would sweeten it.
Just my opinion - there are many ways to use slimfast, after all, it's just one tool among many. If mixing it with water is good for your program then do so! Best of luck with the weight loss - come and visit the slimfast forum, we can always use more folks.
Counting down to a healthy BMI by May 1: