I have read of 3x and 4x as being the minimum before. I also have read the this is measured along the tube surface from the miter to the edge of the fillet OR measured through the fillet from the miter to the center of the fillet's curved outer surface (which would mean the first way to measure would be larger). But i agree, too, that most builders end up with fillets larger then either of theses measuring methods. Also I think most (I know I do) try to end up with an internal fillet in addition of the big outside one. This internal fillet insures that the miter has been fully flowed with filler, so tube to tube contact is complete.

It has been my understanding that the x3 or x4 size is driven by the strength of the filler VS the tube's strength. So if a tube is rated at 100 units of tensile strength and the filler at 33 units then the x3 size is correctly the minimal size. I believe this is why weld fillets can be so much smaller then brass ones, as the weld filler has essentially/nearly the same strength as the tube.

I'm curious what those with real knowledge have to say. Andy.