Adding more milk is definitely not the way to go if I've learned anything lol, I just have a tough time nailing the consistency. I'll make my bechemel really soupy and once all of the cheese is added, it's too thick imo. I've never been able to nail a Mornay, I make them so infrequently my technique is probably terrible haha.

As to the sodium citrate thing, I honestly eyeball it. You don't need much for the reaction to take place. I'd guess maybe 1/3 of a cup(~80 ml) lime juice to a teaspoon (5 ml) of baking soda. So 16:1 ish. It's not an exact science. I usually juice two limes and use a spoon to throw a little baking soda in there.

Another thing you could do if you don't want to buy/use straight sodium citrate is to throw a slice or two of American cheese in with your sauce. That shit is absolutely loaded with sodium citrate. Be careful with how much you use though as it will impart the flavor of American cheese into your sauce and if you use too much it can take away from the other cheeses you're using