Honestly, it's not odd to use batch weight to calculate the amount of scent. If you make any other B&B product -- lotions and such -- you'd use batch weight as the basis without a qualm. It's only soap that has a tradition of calculating additives and water based on the fat weight, not batch weight.
I calculate my fragrance for soap based on 90% of the weight of fat + water + NaOH. The 90% factor allows for weight loss during cure.
I calculate my fragrance for soap based on 90% of the weight of fat + water + NaOH. The 90% factor allows for weight loss during cure.