Calibrate your scale by adjusting the pointer until it is precisely centered on the zero with nothing on the weight-sensing plate. Electronic scales often do this automatically. Dietary scales work well for small wax pieces.
Place your completed wax model, including sprue/gating system (the wax tubes that will create paths in the investment for pewter to pour in and gas to escape. If your model was in something other than wax, melt sculpting wax (available at art and jewelry supply stores) on a double boiler and pour it into your mold, including any pathways left for the pewter to pour in. Remove the wax from the mold and weigh it.
Multiply the weight of your wax by 9.5. This is the specific gravity of pewter in pennyweight, abbreviated as dwt. Specific gravity is an object's density in relation to water that is 4 degrees Celsius, which is approximately 39 degrees F. The weight of your wax times 9.5 is the pennyweight of the pewter you will need.
Multiply your answer by 0.055 to get the weight in ounces if you don't know the pennyweight of your pewter.