MPソープ

finished  = mould volume × base density     (or bars × grams per bar)
fragrance = finished × fragrance %            # % of the finished soap, as IFRA counts it
base      = finished − fragrance − additions
melt      = base × (1 + extra %)

rebatch:
liquid          = grated soap × liquid %
added superfat  = stirred-in oil ÷ (soap × oil share) × 100

• IFRA Standards express every limit as a percentage of the finished product; a supplier Certificate of Conformity states the maximum for category 9 (rinse-off soap) the same way.
• Glycerin melt-and-pour bases weigh a little more than water, about 1.03–1.10 g/mL depending on the base. The default of 1.06 g/mL is the middle of that. — Weigh a filled cavity of your own mould once and use that: it replaces this and the cavity volume in one go.
• Working convention for melt-and-pour: fragrance around 1–3 % of the soap, added oils and butters no more than about 1 tablespoon per pound (≈ 3 %), and dry additions such as clay or exfoliants a few percent. Past these a base sweats, softens, loses lather or does not set. — A habit of the craft rather than a measured threshold; the base maker’s own data sheet comes first.
• Working convention for rebatching: liquid of about 10–25 % of the grated soap’s weight, the less the fresher the soap. Too little and it scorches and will not come together; too much and it stays soft for weeks. — A habit of the craft rather than a measured threshold.
• Oil stirred into finished soap is not saponified: there is no free lye left to react with it, so every gram adds to the bar’s superfat, as a percentage of the oils in the original recipe.