How full to fill it

The empty space above the product is not nothing. It is a measured quantity of oxygen sealed in with a formula you are trying to protect from oxygen.

Headspace is oxygen

Every milliliter of air in the pack carries oxygen, and oxidation is one of the two mechanisms degrading your oil phase and your actives. L-ascorbic acid degrades in oxygen at a rate depending on pH and oxygen concentration, so the volume of air above it is a variable in its shelf life.

Which means fill level is a formulation-adjacent decision rather than a purely cosmetic one.

Why you cannot fill to the brim

Thermal expansion in transit, the volume the closure and dip tube displace, and the practical need for a fill tolerance. Product against the rim also prevents a clean seal, which is a contamination route and, with an induction liner, a failed bond.

So some headspace is unavoidable, and the aim is the minimum that works rather than a comfortable margin.

The airless exception

An airless system removes the question. The follower plate sits against the product with no air column above it, and none enters as it empties.

That is the strongest argument for airless on anything oxygen-sensitive, and it is worth noticing that it solves the preservation problem and the oxidation problem with the same part.

Setting a fill level

Fill to weight rather than to a line, because weight is what the label declares and because product density varies between batches by enough to matter over a run.

Set a target slightly above the declared quantity, since the distribution has two tails and only the low one is a problem. Then check-weigh from the start, middle and end of every run and record it, because a drifting fill is usually the first sign that something upstream changed.

References