A good rule of thumb for white sauces is 1 Tbsp flour to every 1 Tbsp. butter and 1 cup of milk. The Outback Soup recipe calls for a thicker white sauce to make the soup really thick and creamy, hence less milk. If you find the sauce is just too thick for you, add small amounts of milk and allow to thicken before deciding to add more until you get the consistency you like.
Add some Dijon mustard and nutmeg will turn your white sauce into a Bechamel Sauce.
If the first time you make a white sauce you get lumps, don’t worry about it. You just created dumplings! If you really, really need the sauce to be smooth, process it in small batches in processor or blender. Don’t put the top on the blender tightly, better yet take the insert out of the lid or you’ll have a sauce explosion in your kitchen. I’ve done it so you don’t have to!