Brian, congratulations on doing a nice job of practicing fillet brazing! I have a couple of simple suggestions to help you with your journey. I’m guessing that you might want to move the long stem tube around more to keep the walls of both tubes at an equidistant angle from straight up. This means the melted brass will flow towards the crease instead of away from it. This allows you to keep the mass melted just a hair longer so it flows out more and becomes a bit smoother (just before you have to flick the flame off to keep it from running away). Eventually you will be working at keeping the width of the fillet consistent. I also recommend cutting off the used end of your tree so nothing gets in the way of your flame.

If your tubes are both the same size I recommend getting a smaller tubing diameter for the short stubs so it is easier to keep the area around the ears of the miter from getting away. Once you have mastered the technique with an easier braze you can graduate to a bigger stub. I also 2nd Eric’s suggestion of mitering the stubs at angles to get a more realistic practice braze. You might start with 73s before going to the slightly more difficult 60.
The advantage of having a long tube is that it serves as a handle to keep constantly moving the joint to the best angle for fillet brazing. Of course to do this your tube holder has to be tightened to that sweet spot between tight/free where it will move without much effort with your non-torch hand but will stay put when it is in the right position without having to adjust the vise handle. My oldest most beat up wooden tube blocks work best. New blocks that haven’t been broken in can be a bit sticky to move easily.