First off, get ride of that fucking door. Chances are it has some kind of paint on it and is totally screwing with your weld. Weld and paint don’t get along to well. If the door doesn’t have paint, hopefully its a door you can weld (steel or stainless).
If you can’t get the machine turned down enough, try to get your hands on a piece of aluminum and put it under the spot your trying to weld. . .It will act as a backer (so you won’t burn threw & also as a heat sick).
Where ever you happen to put the wire on your welder, chances are there is a chart that may say some kind of ball-park settings to start at. The door is probably something like 20ga - 16ga, depending on what kind of door this is.
One other thing you may be able to try is welding down hand. Flux-core doesn’t like this either, but for what you want to use it for I think you’ll be fine. Just stand the metal up, start from the top and drag it down. Good luck!
And if none of this helps, go to wal-mart and buy some stenciles & paint your letter onto your door.