Weighing, not measuring
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There is no volume measurement anywhere in this craft, and one of the weights has a safety consequence.
Everything by weight, including the water
Oils, lye, water, fragrance, additives, all by weight.
Oils have different densities, so a volume recipe is not reproducible and cannot be scaled.
And the lye calculation is expressed entirely in weights, so mixing in a volume measure breaks the arithmetic the recipe rests on.
Lye is the one that matters
Every other ingredient being slightly out produces a slightly different bar.
Lye being out produces either a lye-heavy bar, which burns, or a badly superfatted one, which goes rancid.
So weigh it last, weigh it carefully, and check the figure against the calculator output before it goes anywhere near the water.
And weigh it into its own container rather than adding it to a tared vessel of water, so a slip can be corrected.
Two scales
One with capacity for the oils, reading to a gram.
One reading to 0.1 g or better for fragrance, color and small additions.
Check both against a known weight periodically. A drifting scale is invisible and it moves the lye figure.
Recording it
Write the actual weights, not the targets, and the lot of each oil.
Oils vary in saponification value between lots and between suppliers, which is why the superfat margin exists and why the lot is worth recording.
And record the calculator settings, superfat percentage and water amount, because a recipe without them is not reproducible.