Is there a "rule of thumb" about how much tubing is supposed to be sticking out of the flaring tool ??? I thought it was like half the diameter of the tubing... so for 3/8 tubing, you stick out 3/16 ???

Reason I ask, making some parts up today for some oil lines, I couldn't get a "straight" flare to save my life. Off to one side, every time. I ended up starting the tubing flush with the tool, and making a small flare first, then putting a bit more tubing out, and finishing the flare. Maybe my tool is bent... (get yer minds outta the gutter!!)