Interesting -
I also make a white sauce to pour over the cut potatoes before baking them - but I make a thin white sauce -
for a 9x13x2 pan - I use about 4 T butter, 4 T flour, and about 4 cups of milk. It is kind of runny when it comes out of the oven, but it thickens up quite quickly. When pouring the white sauce over the potatoes, I only fill it to about 1/4 inch from the top of the pan. It bubbles while baking, and needs some room to do so without cooking over.