I think you're getting it now. This is for the Lard soap recipe, the rule of thumb ratio is a estimate, range of 30-40% lye solution to fats/ oils. I don't use the rule of thumb for other soaps because of the various oil types.
using: 1. Lard, 2.purified water, 3. Sodium Hydroxide. Let's say you have a lb of Lard. You melt down a pound which is 16 oz .You might score an ounce or two more in liquid volume, so 17-18 ounces. You apply the rule of thumb ratio. (1 part lye, 2 parts lard) let's say 7-8 ounces. However, You have to make your lye. No calc..np. Apply rule of thumb ratio. This will be about 5 ounces of water to 2.5 ounces of sodium hydroxide after it's saponified I add sea salt before pouring. I hop this helps, didn't mean to make you work so hard lol. Just use the calculator if this still doesn't make sense or if you doubt it. However, if you ever stranded in the wild and have to hunt for your food and make your own soap keep my ratio formula in mind.