This should automatically update whenever you change any of these fields. I've only tested it in firefox though, so I apologize if it doesn't work in whatever browser you're using.
If you'd rather use a unit other than ounces, just pretend it says what you want instead of "oz." I only put ounces there because it looked weird with no given unit.
If you're unsure about what values to give, here's a table of values that I basically got from wikipedia. If you're trying for something specific, you'll likely want to do your own research :-)
|Low fat milk||1%|
|Reduced fat milk||2%|
|Half and half||12%|
If you want the fractions to be less specific, you can control that here:
(If clicking one of these boxes appears to have no effect, try clicking somewhere else on the page right after.)