What I would do is mark my full level, and replenish to that level each night and use gallon bottles to replenish so I could monitor the amount being used.
For smaller indoor setups, I would keep track, if I had a 20 gallon res. setup, I would fully replace the
nutrients every 2 weeks OR if I added a total of 20 gallons to the res. to keep it level. Which ever came first.
Plus...get a TDS meter, it'll let you know how much of your nutrients are used. Mark your starting number, and check it each time you top off your res.