I'm telling you, I just about blew a brain gasket trying to figure out how to adjust my recipe to accomodate the reduced salt amount that I use so that my batter would fill my mold nicely.
The problem with salt is that you cannot do a 1:1 swap with the oils because salt and oils have different densities, meaning that one pound of salt will not take up the same volume of space as one pound of oil would.
I eventually decided to take the easy way out- I used my slab mold which has sides tall enough to more than accomodate any overflow if I make my normal 2.5 lb batch in it. It's a handy mold. If I want to, I can pour either 2.5 lbs of soap in it in a single horizontal layer, which gives me 9 bars of soap at 1.25" thick, or I can pour 5 lbs of soap in it, which doubles the depth of the batter to give me twice as many bars of soap (and twice as much cutting to do!).
Well, anyway I made my normal 2.5 lb recipe, added the salt, poured the batter into my slab and then let the chips fall wherever they might. My finished bars were somewhat thicker than normal, but not gargantuan or anything like that. I figured that I only needed to reduce my oil amount by only an ounce or so for my bars to come out at about the same thickness as normal, maybe slightly heftier, but that's okay since I like heftier bars anyway.
Another easy way to figure it out is make your normal size batch, add the salt, pour into yor designated mold, then pour any extra batter into an extra waiting mold and weigh. However much it weighs, just reduce your batch size by that much the next time you make salt bars.
IrishLass
