For dried beef gravy I use 6 TBSP flour to 6 TBSP butter to 3-1/2 cups of milk. Due to my grandson's medical condition, I need use xanthum gum instead of the flour & heavy cream rather than milk. Can you suggest proper proportions? I would appreciate any assistance you can offer.
I'm going to guess that your grandson is "gluten intolerant", and that you did some research and discovered that gluten-free baking often uses xanthan gum as a substitute for the structural effects of gluten in wheat flour.
Which is true, but not relevant to your situation. When you use flour in gravy, gluten formation is unwanted. (That's one of the reasons you cook the flour before adding the liquid: it prevents gluten from forming.) The important textural aspect of the wheat flour is the starch, which thickens the gravy.
There are other sources of starch which can thicken liquids like gravy. Cornstarch and arrowroot starch are the most common in American cuisine. You can use xanthan gum to thicken liquids, but the result has an extremely slimy consistency, particularly if dairy is also used. Trust me on this: you will not want to eat a gravy which is thickened with cream and xanthan gum.
Cornstarch is a slightly more powerful thickener than wheat flour, so I would use about 70% as much. It does not need to be cooked in a roux; thoroughly mix it with a bit of cold water and then pour it into the gravy while mixing rapidly. The appropriate amount of xanthan gum in this case is 0%.
Use .25 to 1 percent xanthan gum to thicken. Once you get above 1.5%, you might find the texture unpleasant. Start on the low end. Give it some time after the addition of xanthan, before you decide you need more. It is easy to over-do it, with the result being a snot-like texture. It will also be more viscous at rest, than it is when stirred. Rather than using cups and spoons, it is easiest to weigh your total liquid, then calculate the percent by weight.