Hello Richard: The wax weight multiplied by the “specific gravity” of
the metal you are casting in multiplied by a factor of 1.1-1.2 for the
sprue. In this case , Sterling silver has a specific gravity of 10, so
wax weight times 10 times 1.2 equals metal needed. I use a vacuum
caster. Many of the jewelers supply catalogs have a chart of specific
gravities of metals in the back.
Michael R. Mathews