Sauce – How to make a mustard sauce for salmon steak

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.

Per SAJ14SAJ's comment, the suspension in a traditional veloute relies on the gelatin to keep everything in place. Stocks are naturally rich in gelatin, due to the bones and connective tissue used to make them. Using a base to make the sauce is probably not providing the gelatin necessary. Other rich liquids frequently do separate when simmered for a long time (think curries and gumbo), so it's to be expected that your emulsion will break down eventually. I think your solution is to either use a real stock instead of the base, or perhaps fortify your liquids with a bit of gelatin before adding the roux (or barring those, cook only until your starch is gelatinized).