A “roux” and what are its uses

My understanding is that the ratio shifts from 10:1 (liquid to roux) for a pale roux, up to about 5:1 for a nutty brown roux, with a somewhat linear relationship between the two extremes. A dark brown roux has very little thickening power, mostly they are used for flavor more than actual work-a-day thickening.

The problem may not so much in the roux ratios, but in other ingredient variations. For instance, if you are using a traditional 3:2 flour:fat roux, and your fat is butter, make sure you are using clarified butter, or that you are allowing most of the water to cook out of the butter before adding the flour - excess water in the roux will break down some of the starches and reduce the thickening strength.

Finally, there will be some variation no matter what - you can always make a little extra roux in a second pan to integrate if necessary.

I would think that if there were a change, that the sauce would be thicker, due to water vaporizing and the fridge that tend to dry things out.

So, I think it's in your head, but of course I could be wrong.

The one reason why it might appeared so, is because the sauce will become thinner if you reheat it, compared to the consistency it had when it was in the fridge or at room temperature.