For an example, let's say I have a demijohn with 20L of wort that is 50% of the way to becoming beer. Then let's say I add 80L of freshly brewed wort into the demijohn. Intuitively, you might expect the resulting wort to be 100L of wort that is 10% of the way to becoming beer, but is this how it actually works? Does it just reset the timer, or maybe adopt the timer of the newer liquid, or does it combine them and make a new timer based on the quantities and timers of both sets of liquids?
This conversation probably applies to wine/vinegar too.