Even more important than these channels is venting of all of the 'dead ends' within the mould. As the metal flows in, the air inside the mould has to go somewhere otherwise you end up with 'rounded' not 'sharp' detail. Have a look at a Wiking cast metal ship to see what I mean.
(The alloy used is critical; I believe Wiking used lead which rarely sets to a sharp edge.)
I use a fine drill bit in a pin chuck for this. As the metal flows in the air is vented out. Often wisps of metal can be seen on unpainted casting where the metal has followed the air into the drilled vents. 
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Besides all the other things like how to cut the air channels or vents in the silicone mold, the alloy mixture is probably critical. Think lead, tin, antimony and maybe zinc are all used, possibly not in same alloy. Previous Message
At one of our New York meetings Rudi Kraus brought a worn mold for us to look at. I remember he stated that some additives were added to the metal to allow it to flow more freely in the mold. One of the additives was antimony, but as this was probably Neptun's trade secret he did not give any percentages of the additives.
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