Algodoo

shows where moons can be.
the "density" box only affects the red circle
the "radius" box affect all 3
type in what you want for radius/density and it will be automatic
the center circle is the moon/planet, and once u size it u can decorate to make your systems!
the red circle is inside the rosche limit, where moons break apart from planet's gravity
the blue circle is the moon zone, outside it and the moons fall away
(i know it depends on outside gravity please dont do anything bad about it ill make that in ver. 2.0)
it works for stars too
so, inside the blue, but outside red is the moon zone.
note: calculate your moon's sub-moon zones, which should never overlap, otherwize your moons will pull eachother out of orbit.