I'm confused about what is used to determine how many "pounds" of soap my batch makes.... Do I add up the weight of all of the ingredients that goes into it...
I'll take a stab at answering the question I think you're asking --
When soap makers talk about a batch of soap or the ingredients in the soap, they often use the weight of the fats as the standard of comparison, not the total weight of the entire soap batch. They'll usually say something like "the batch is made with 42 ounces of fat". They are much less likely to say "the batch weighs 64 ounces total."
Why? The total weight of fat is the largest ingredient in soap. It is also the one ingredient in soap that remains a constant. For every pound or kilogram of fat, soap makers will use more water or less, more scent or less, no or some or a lot of additives, and so on. Even the lye (NaOH) weight can vary depending on the soap maker's choice of the superfat percentage.
All these choices means the total batch weight can vary quite a bit. That makes it hard to use total batch weight to compare batches of your own soap or compare your soap with another person's soap. A more consistent way to make these comparisons is to use the total fat weight. It's not perfect, but it's better than the other options.
Bakers do the same thing -- they will use the total weight of flour as their basis, not the total weight of the bread dough or cake batter. Look up "baker's percentage" for more info.
FWIW, I wrote an article that gives step by step instructions and the math background for the "0.40 Rule" and its metric equivalent. These are ways of estimating how much soap batter a particular mold will hold. The answer you get is the ounces or grams of fat needed for the batch, not the total batch weight. See --
Estimate batter to fill a mold | Soapy Stuff