Yeah, that's the thing about it. If you look at a real one you
can't really put your finger on the paint and say the demarcation
starts here and stops here, that's the whole idea behind soft
edges. A lot just depends on the guy who is spraying it, how
close he gets, how careful, etc.
I usually guess that from soft edge to soft edge is somewhere around 6"
more or less. On 1/32 scale that's about 3/16" which is pretty
easy to do. At 1/72 scale it's down to 1/16" and at 1/144 it's
around 1/32". More is not going to make it look bad as long as it
isn't a lot more. If you get up around 1/16" that equates to
about a foot on the real plane, and that would be pretty wide.
Also, the demarcation will be a lot more pronounced (i.e. visible!) in
high-contrast areas such as between dark green and tan.
I think your biggest problem is actually going to be the size of the
spray droplets. Unless the paint is really good and thinned
properly the spray "Dots" are going to look huge at that scale.
Play around with thinning, pressure, and distance from the surface
until you find something you like. If you use acrylic I'd
recommend a good bit of retarder in the mix to insure the spray doesn't
dry before it hits the surface and has time to level out. It's
probably going to take some trial and error so the milk jug would be a
good place to start.