The estimated resin to wax is the resin is about 10% heavier then carving wax. I multiply the resin model x .90 to get the equivalent wax weight. This is for red or red/cherry mix. Have been using this for 6 months. Go back to page 1 and see the posted specific gravity (SG) for most metals u might deal with.
I use same ratio I would use for wax and they seem accurate enough. below link has different karats and metals
http://www.hauserandmiller.com/reference/melting.html