That formula calculates the volume for an average batch of soap. It assumes that the average soap recipe uses oils whose weight coefficient is 40% of the volume of the soap in cubic inches. If you use less dense oils or a larger volume of water for your recipe, then you will have more soap than the formula indicates.
If you have a base recipe that you use consistantly, you could tweek the formula for your own use by melting your weighed oils and recording their volume in fluid ounces. Mix your lye solution and record its volume in fluid oz as well. Add together for total volume in fluid ounces, then multiply by .554 to get volume in cubic inches. Then take the original weight in oz. of your oils and divide them by the volume in cubic inches. Use the resulting fraction instead of 0.4 in the mold formula.
Heh! 8)