Dan- Internal fillet/penetration is a factor of the brazing, not so much the miter. Getting the junction of the tubes hot enough so the filler can flow between them. (Without overheating). having flux on any surface you want the filler to go to is what i do. I also like to have a reserve of flux near by, meaning I'll flux beyond the area of filler placement. So, yes, I flux both tubes maybe a 3/4" or more past the junction. The post i just did of the SPT shows a tacked pair of joints. Note how far up the tubes the flux is. the inside surfaces of the non mitered tubes were also fluxed. This is why having sufficient vent/soak holes is a good idea. So the inside flux can be soaked off and the insides can drain/dry. Andy.