You can just add up the weights, but remember that much of the water would cure out. Usually, I account for the mass of oils plus the (mass of dry lye twice) for a final cured weight.
I double the mass calculation for dry lye for this estimation because I expect that much water to be retained in the soap after cure. It is possible that more or less water will remain depending on atmospheric conditions where the cure happens or where the soap is stored afterward, but an equal amount of water as lye is reasonable, I think.
Do any of our more experienced soapers have anything to add to this?
