Basically, I use 2 parts low fat sour cream to one part mayonnaise (Hellman's Light) and just enough Dijon to add a piquant note (sometimes just a squirt, sometimes a tablespoon or more), plus kosher salt and freshly ground pepper. To start, you might try two Tablespoons sour cream to one Tablespoon mayonnaise and 1/2 teaspoon Dijon. That will give you the basic mix to taste for other seasonings before you increase amounts.
I mix all of these in a separate bowl to get the flavor and consistency balanced, and then I add them to the egg yolks which are broken up but not mashed yet. Mixing in the creamed ingredients will take care of the mashing.
Adding that extra bowl and step has saved me from egg filling that is too soft/loose many times, since I can add, mix, evaluate, and add more. Adding less is not an option once it's mixed with the egg yolks.
To transport a large number to a party. I take the egg whites and filling in separate zip bags and assemble them on site by removing extra moisture from the whites with a paper towel and clipping a corner of the filling bag to turn it into a pastry bag. Takes just a few minutes and keeps the filled eggs from getting watery. A sprinkle of paprika or dill on top is the only other thing that I do.
Other additions, of course, will change the flavor and the texture, but as I said earlier, the point of such a simple mix was to please varied tastes at a potluck.
I apologize for not being able to give you more specific about amounts.
