I have made salt bars before, and adding salt to the batter certainly makes the batter go a fair bit further.
I think most recipes call for 80% of your oil weight in salt.
So, if you have 1kg oil weight, add 80% (800grm) salt, so you would end up with 1.8 kg, of soap, almost twice what you started out with!
With that in mind, maybe scale down the original recipe?