I think you can start getting some ideas by reading this Rabbi Kaganoff article. It won't answer every type of instant food around, but it should give you some ideas of the main rules. Before starting, we have to assume that the base ingredient has already been cooked, so instant coffee, would be OK. I'm unfamiliar with how powdered milk is made. Of course, if you add cold water to any instant item, there is no concern of cooking it, anyway.
The bigger problem that Kaganoff's article addresses is the melacha of losh or kneading (forming a "dough"). Generally, in the article, he claims that if the "instant" items are small and adding liquid forms a sticky dough-like mass, then you have a problem. A similar problem happens if you form a "batter".
Applying these rules, from my best analysis, using milk powder doesn't seem to be a problem because it doesn't form a dough. Same thing to be said with soup powder. But instant mashed potatoes would probably be a problem. Now that I think about it, tahini has a strange character. It first becomes a "dough", but as you add more water, it thins out. So, I'll try to re-ask my rav why he believes this is still OK, but why mashed potatoes may be a problem. To me, they both seem to act similarly.