I might be a bit crazy in this, but then I've never painted an entire army of figures wearing long coats, kilts or skirts; only individual minis or squads.
What I do is fade the bottom, from the colour it is on the outside, to black at the center of the coat/skirt. It gives the (rudimental) impression of depth, which I find more pleasing than a very stark line between the outside colour and black bottom. If we're looking at white or very lightly coloured outsides, I usually do not fade to black, but to a medium grey or brown.
I've also seen miniatures where the bottom of the filled in coat/mantle/skirt actually bulges downward, probably an artifact from the way they were cast (preventing undercuts?). I've never painted one of those, but I reckon my technique would not work there. In fact, I would not know what technique would achieve an acceptable result in those cases, and I'd probably end up cutting and filing such a bell bottom flat or even concave...