This is a question that I am sometimes asked by people, and the answer that I give to them is described in this blog. It is not enough to merely say: “You need just enough filler metal to fill the joint.” Then the next question usually becomes: “Well, how much is that? How would I calculate what is 'just enough’?” Here is my suggestion.
First of all, as shown in Fig. 1a, you should calculate the “average” volume of the joint that is to be brazed based on the “average” thickness of the brazed joint you expect to be brazing. Hopefully, the joint clearance will fall somewhere in the range of 0.001-0.005 inch (0.025-0.125 mm). When you have determined the average (i.e., “median”) clearance you will be using, multiply that answer by about 1.5. That answer should represent the maximum amount of liquid brazing filler metal (BFM) that you want to have in the joint (Fig. 3).