As Andy noted you "should" end up with an internal fillet. If the mitres are really tight, as they should be, one thing I've done is file a couple small notches in each side with a triangular file to insure the filler will flow to the inside. It helped in practice joints, but you never know about the finished product. If it keeps sucking up filler- you're good. One thing I don't care for is the feeling that I need to add a little more heat to make certain the headtube is hot enough inside the joint. You can see the heat spread from inside the headtube, but it's always a real juggling act for me. Just takes practice, and the more you do the more comfortable you get. My problem, as a hobbyist, I don't get to do enough frames.
That said- I don't like the appearance when the downtube and headtube are close to the same diameter. You have a nice radius to the fillet transitioning to a "flat" on the sides. It just don't look right to my eye.