If you have a print that started to "Fill-in" a solid area as is the example of the Secret Butterfly Box Drawer (Attached .stl)
If you print this example on Fast with layer thickness of 0.3mm then you will get to layer 11 as per attached L11.png.
L12.png is the drawing of the layer on top of L11.png
You will notice there are no "outline" support for this new "Fill-in" layer that started in the middle (the top of the key area).
Therefor the new fill in layer that was created in the middle of this print, does not have any support below it on the edges.
It would have been better if layer 11 had in outline around the key like L16 but obviously the middle section also filled in with in this case 20% fill-in.
This will give the new "Fill-in" area of Layer 12 a solid foundation to bind the edges to and will improve all layers above it.
This new layer 11 outline will sit on all the various fill-in layers of layer 10 and therefore will have a very level structure.
An alternative to this would be to create a solid layer just before we add a new split fill-in. Therefore in this example, layer 11 would be a solid fill-in which will allow for the various fill-in section of layer 12 be all have support under it.
This second suggestion would be additional to the "Solid every x layers" option currently existing.
If detecting these layer are difficult, we could perhaps start with a text box, where the end user can specify which layers he want solid with a coma separated.
In this example, I could select something like: 11,15,18,26 and it will print layer 11 solid fill-in, as well as layer 15, 18 and 26 all solid fill-in.
Hope this helps improving Simplify3D even more. Thanks for a great product.