How to estimate amount of all purpose flour for roux? I was making some roux for white sauce for pasta and followed instructions to the dot. (All purpose flour == butter == 4tbsp. 2 cups milk.) But the milk was very very thin (the only variant we get around here) and I ended up with a very thick sauce.

Is there any way I can estimate when to stop adding the flour (which I predict the culprit for any inconsistency in the sauce)?

I usually


Heat butter
Add small amounts of flour and stir while adding some milk simultaneously.
Typically, you go for 1 tbsp flour, 1 tbsp butter for each cup of liquid. 
You may have to adjust slightly, but that's a good starting point.

After the quantity of roux, the next factor to thickening strength is cooking time.  The longer you cook the roux, and consequently, how dark it gets, the less thick the final result will be for a given quantity of roux and liquid.

For a white sauce (i.e. Bechamel and Bechamel-based sauces) you want the roux to stay more or less white.
For a Gumbo, you cook it until it's light brown (sometimes called red).