Wine4Me:
I suggest that the next time the fermenter is empty that you measure 23 litres (6 US gallons) of water into it, and mark the level on the outsides. Then you have a target when starting a kit.
Or you could buy a good quality 23 litre kit (spagnols Bolero or Premier Cru, or Vineco Vinterra, or Mosti Mondiale whatever), pour it into your primary, and then mark the level.
Steve