I like this definition as well. Tategoi is not always a tosai, but very well could be a 4 or 5 year old. I also take into consideration who's tategoi. Sakai's tategoi are drastically different than my personal "keep fish'' in my own pond vs my culls.
It is not a standard written in stone by any means. I do find that sellers advertising fish for sale as tategoi confusing in that they are getting rid of their keep fish - yeah right.
Aquitori, your question asks about tategoi, but another question could be what is your personal criteria for selecting tosai? or Nisai? etc. That would be quite a different set of answers.
For my personal pond collection, my keep fish criteria in the past have been set around target fish load in the pond. For the benefit of the others in the environment some had to go. I could see a breeder making similar decisions based on available space and food resources so one year the culls might have a much higher quality than a different year for us buyers of the unwanted koi.
Rick