Is it possible to make evaporated milk using powdered milk?
The primary difference between evaporated milk and a strong milk made from instant dried milk, aside from that great caramel-y taste, is the fat content.  Most instant dried milk powders are fat-free.  The fat in the evaporated milk is an important component in a lot of recipes.  However, that can be overcome with the addition of some margarine with the milk and water.

There is a nice discussion of this option with some recipes here:  

http://www.postandcourier.com/article/20080525/PC1206/305259953

Perhaps that will give you ideas and a starting point for judging ingredient amounts.