Wow, so many volumatic problems......and all the multiple checks, if I checked the volume of my carboys, one by one, one liter ar a time, the corona virus would have passed before I was done. Instead, why not take advantage of the wonders of the metric system? All you really need is a good weight scale to determine volume, right???? 23 liters of water weighs what? Rightttttttt !!! So you could weigh any vessel, add 23 KG of 4°C water to it, and that’ll be your 23L mark.
You’ll soon find, using your newfound volume tool, as
@sour_grapes alluded, many quality carboys hold varying amounts of liquid, most are slightly over 6 gallons / 23L when topped up properly. When I was making kits, and I made hundreds of kits, I made 6 gallons of must, period, as the manufacturer directs. It normally resulted in the need to top up down the line, which was always done with wine, and sometimes demanded several bottles. This is a “Best Ptactice” we’ve been sharing here for many years. No one is trying to convince you to change, you may use water if you like.