We actually do not have any direct experience in making Casein from skim milk. We did, however, research how much Casein powder is gained from 1 gallon of milk and it was a minimal amount (something like 30g).
The beauty of working with the metric system, is that 1ml of water weighs weighs 1g. So maybe this will help in your explorations.
Our casein powder does not have any emulsifying additives and yields a very powerful glue strength, when used in higher concentrations.