I’m not sure how anyone could make an meaningful estimate on that. At that stage I can’t see that the fish is growing in size, simply fat deposits that will increase and decrease. We’re no longer talking about growth rates of growing year classes. There are so many factors that will impact on this, such as food availability and quality, climate, flow, genetics etc etc.
In a closed environment like a lake, where things are less subject to environmental factors things are probably a bit more predictable. So the carp boys can make better assumptions based on averages. For a barbel in a river, I don’t think this is possible.