I have accepted whatever superfat the calculator suggested for eight batches - what does the number actually change?

Superfat is two things at once, and separating them is what makes the number choosable.

It is an insurance policy. Your oils have a stated saponification value, your scale has a tolerance, and the lye takes up moisture from the air. Superfat means using slightly less lye than the arithmetic says, so that even with all those small errors stacked the wrong way, there is no free lye left in the finished bar. That is why almost nobody goes to zero, you would be betting on every measurement being perfect.

It is a property of the bar. Unsaponified oil left in the soap makes it feel less stripping on the skin, and contributes to a creamier lather.

So the trade at each end:

Low, around one to three percent. Harder, longer lasting bars with more cleaning power. This is why laundry and utility soaps sit down here. Less margin for error, so it wants accurate scales and fresh oil values.

Middle, around five percent. The default, and it is the default for good reason: enough margin to be safe, enough free oil to be pleasant.

High, eight to ten and above. Gentler feeling, softer bar, shorter life, and a real increase in the risk of the free oils going rancid over time. Above ten it starts trading away too much.

That rancidity point is the one people miss, and it connects directly to a problem you may already have seen.

The thing that changed how I think about this: which oils you use decides more than the superfat number does.

Superfat is a percentage of your total oils held back, but you do not get to choose which molecules stay unsaponified - in a normal cold process batch they are distributed across whatever is in the pot. So a five percent superfat in a recipe that is mostly hard oils behaves quite differently from five percent in one heavy with soft oils.

Which means if you want a specific character, change the recipe rather than the number:

Harder, longer lasting - more of the hard oils. This does more for bar longevity than dropping the superfat.

Creamier, more conditioning: more of the oils known for that, rather than pushing superfat up to compensate for a recipe that is too cleansing.

Bubbly lather: again a recipe question.

The practical order I would suggest: get the oil blend making a bar you like at five percent, and only then adjust the superfat, in small steps, one variable at a time.

And note that a high superfat is often a plaster over a blend that is too harsh. Fixing it at the source gives you a better bar that also lasts longer, rather than a soft bar that goes off in eight months.

Eight fine batches at the default is not a failure to learn. It is the default being sensible.

Some oil left unsaponified is the definition and it tells you nothing about what to pick.

Two practical consequences worth knowing before you start moving the number, because both have caught people out.

High superfat and rancidity. Those orange spots people find on old bars are oxidised free oils, and the more free oil there is, the more there is to oxidise. Pushing superfat up shortens shelf life, and it shortens it most with oils that are prone to going off in the first place. If you make in batches and use them slowly, this matters more than it does for someone who makes monthly.

Fragrance and additives eat into your margin. Anything you add that contains oil is effectively adding to the superfat without appearing in the calculation. Same for milk, cream and anything with fat in it. If you are running high already and add something rich, you can end up well above where you intended and get a soft bar that will not release from the mould.

Also worth being deliberate about: if you change superfat, change nothing else that batch. The whole reason people cannot answer this question for themselves is that they change the number, the fragrance and an oil at the same time and then cannot tell what caused the difference.

Make the same recipe twice at five and eight, cure both properly, and use them side by side. That comparison teaches you more than any recipe's stated number.