I was wondering what is an easy and effective way of airbrushing a soft edged demarcation line between the underbelly and the upper fuselage?

Since you are airbrushing, it can be free handed. Just do a little practce until you are confident of your abilities. Shoot your bottom color first as it is usually the lightest. Easier to spray dark over light than the other way around. Good luck. "Q"

Hi there. Try pulling your air pressure down on your compressor. I use maybe only 10 psi or a little more when "drawing" the soft edge camoflauge onto an aircraft.
The reduced pressure helps eliminate overspray. Hope this helps. All the best. Russell

You need to be able to paint the finest line possible, to do that you need to get in close. Getting in close causes it's own problems but if you thin your paint more than usual, I thin mine about 70 thinner, 30 paint, but this is not set in stone. Drop you spraying pressure, as Russell says, but I drop mine even further, sometimes. Practice and gain confidence and you will be fine. When you are set to go do a few dry runs first, the reason for this is that you may have to be moving the model as you are moving the airbrush, because of the compound curature of an aircraft. Another tip you might be able to employ is to have a couple of practice runs actually on the model. Simply start close but away from the actual demarcation line, then another a tad closer to the final position (more goes if you what them) then go for it. I promise, that if you practice a little and get the paint consistency and air pressure right, you won't find it difficult.
There is another way and that is to use "blue tack" sausages, or "white tack" as here:

If you role the "tack" out thin and even you get a nice tight featered demarcation. This was a bit of an experiment, along with other models, but I have gone back to free hand painting demarcation lines as it's far quicker.

HTH.

Mal