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I'm afraid I'm away from home so cannot check my box to see the quantities but I can tell you that (at least for my brand) you don't reconstitute the powder and water but add the powder to the dry ingredients and the water to the wet. I have the same issue as you that I often end up wasting the large bottles/cartons of buttermilk so I have found the dried buttermilk works as a good replacement.

The problem of "wasted" buttermilk can be overcome by frezzing it in useable quantities, or making your own 1 tablespoon of white vinegar or lemon juice plus enough milk to measure 1 cup. Stir, then let stand for 5 minutes.. works well for me ;-) Shame they don't sell it in 300ml cartons..!

I have used both the buttermilk powder and the lemon juice and milk method. When using the powder I always mix it in with the dry ingredients and then add the water to the wet ingredients. Way back when I first tried the powdered buttermilk I tried to premix the powder with water and it was always a hassle so when I saw an instruction in some recipe to add the two separately I started doing that and never looked back. I grew up using the lemon juice and milk mixture because we never had buttermilk in the house when I started baking and it always worked great. Just recently I was reading a Cook's Illustrated article with a recipe for a quick bread or muffins and they found that they got the best results with using the lemon juice and milk mixture versus either fresh or dried buttermilk. I can't remember which magazine or book I saw the article in but it made me rethink buying buttermilk when I always have milk and lemon juice on hand, especially now when grocery shopping is no longer as fun or easy as it used to be.

I found the same problem when I first bought buttermilk powder. After some sleuthing online and use in recipes I've found that 1:3 is the right ratio (1/3 cup buttermilk powder + 1 cup water), and I add the powder with the dry ingredients and the water with the wet.

Also, don't worry too much about buying a quart or litre of buttermilk. It keeps way past the due date in my experience, and confirmed by Deb Perelman of Smitten Kitchen. 

And it tastes great just as a drink! Seems like a lot of people don't drink it, but my family always has ...

A Canadian powdered buttermilk !? I didn't know such a thing existed. And I've never noticed such a product in an Ontario store. I've always purchased it in the States when visiting there...Wisconsin made Saco is the brand. Their instructions are 4 tablespoons to 1 cup of water mixing the powder with your dry ingredients and the water with the wet ingredients. If I happen to have buttermilk in the fridge I use it up substituting it in baking recipes calling for milk. And drink the buttermilk....blech ....no not ever lol...though my mother loves it.

Lol I love drinking fresh whole milk bulgarian style buttermilk.   Buttermilk can be used for cooking 1 1/2 months beyond expiration date.  I buy and use buttermilk a lot and learned to tell when they are no good by appearance, smell and taste.  

Thanks for the brand and source.... I'm going to go look right now! For buttermilk powder ....and for vanilla beans, another rare (and mind boggling expensive for such pathetically withered and shrunken condition) find in my corner of Ontario.