Scott Epstein otherwise known as 'Einstein' demonstrates a nice tight transition from a Runner's Stance Arm Triangle to a top side Triangle.
As he says for whatever reason you can't finish the Arm Triangle you lock up his head and go to like a Gansta Lean position. This allows you to get the leg that you had knee-on-the-stomach with over his shoulder to set up the triangle. Here he keeps everything tight by grabbing his ankle to pull it into Triangle position.
At this point Scott covers a little detail that will help you get your foot ankle deep behind your knee to lock the Triangle up nice and tight.