Headspace and why it matters
Updated
The gap between the sauce and the cap is a working part of the package, not slack.
What it is for
Room for the product to expand and contract with temperature without stressing the closure. Room for the vacuum that forms as a hot filled product cools. And enough space to shake a sauce that separates.
A typical fill leaves a small but deliberate gap. The exact figure belongs in your process, because it interacts with hot fill and with the container.
Too little
A bottle filled to the brim has nowhere to go when it warms in a delivery van. The closure takes the pressure, the liner distorts, and you get weeping around the thread that looks like a leak because it is one.
It also makes the bottle impossible to shake, which matters for any sauce that separates, and it fouls the neck during filling.
Too much
Excess headspace is oxygen, and oxygen in contact with the product drives the color change that turns a red sauce brown. It also looks like short measure to a customer even when the declared quantity is correct.
On a hot filled product it weakens the vacuum, which is what holds the seal and signals integrity when the cap pops.
Getting it consistent
Fill by weight to a target rather than by eye to a level. Weight is the thing that scales across bottle variation and viscosity change.
Check a bottle from each run against a reference: a filled bottle you were happy with, kept on the bench. Eyeballing headspace drifts across a long filling session, usually upward as you get faster.