Why wouldn't you just use it on a one-to-one replacement basis? You can replace the water called for with a box mix with an equal amount of milk and the cake will turn out OK.

I remember when Pet and Carnation cans were just a bit bigger.  My pumpkin pie recipe calls for both evaporated and whole milk.  12oz of canned milk and the cans were 13 oz., so I would sip the first ounce out of the can.  Now we are lucky to get 10 or 11 oz per can.  I buy the smaller 7 oz cans that used to be 8oz and use it in my macaroni and potato salad dressing.

While it’s true that evaporated milk really took off in the Depression, I think it was shelf-life and shelf-stability, rather than cost, that made it so important.  The government was trucking this stuff all over the country to people who needed it, and there was no way they could have done that with fresh milk.  And maybe more importantly, country people went to the supermarket infrequently, maybe never, and only a few of them owned a milk cow.  They were probably happy to stock up on evaporated milk.

Similarly in Mexico and Central America, maybe South America, too, canned milk is still just about the only milk used for baking.  Fresh milk in that climate was traditionally not an option for the general public.  Of course, milk consumption was unknown there before contact with Europeans, and most Latinos are lactose intolerant, so it’s use is strongly associated with recipes from the Spanish convents.  The nuns used real milk, and their cheeses and milk-based desserts have always been famous.  But as baking became an option for household kitchens in the 20^th century, ordinary people turned to evaporated milk.  Fresh milk has no future there, either, because over half the population is lactose intolerant.

Sorry I can’t help with the Carnation recipe.  I’ll take a look at some Latino recipes and see what I can find.  There may be something still in use.