FYI to those interested, the clearance between the lug and the tube depends mostly on type of lugs used. If the clearance is on the tight side, 2-5 thousands, a high silver content brazing material like 56% is commonly used (by custom builders anyway), for looser fits, brass is typically used. As Richard mentioned, it's the capillary action that pulls the brazing material into the joint after it turns into a liquid so the diameter of the wire rod in not that important. The material is fed in from the outside and the capillary action pulls the material into the gap formed by the lug and the tube thus there is no need to make a coil in the inside.

Hope this helps.