It might be easier just to decide how many bars of soap you want from each mold and go from there. Example: you want 12 bars of soap. Your soap is 1" thick so your mold would have inside measurements of 3.5 wide x 12 long x 2.5 inches tall. Your volume would be those measurements times .4 (3.5x12x2.5x.4) so you would need 42 ounces to fill the mold.
If you usually do 35% water, then you multiply 42 oz by .35 to get 14.7 oz of water; the rest would be your oil weight: 42-14.7=27.3 oz oils
This is just a general formula, but hopefully it will give you a starting point.