Maybe I should describe it better - it is a special tax model - you pay a flat fee if you make less than X and that flat fee turns out to be ~5%. When you cross this threshold you no longer qualify for this model at all - you have to convert to standard taxation - I would need to increase my revenue by 20% just to break even and the extra hassle of doing that would mean that I need to increase it by 30% (at least) to be wroth it.
So in my situation it just turned out to be a good limit where if I negotiate 10% more I just convert that money = free time for other stuff. I don't plan to stay below this limit forever (in fact next year I plan to go over) but the concept of having a target income goal and then letting the rest of your time go to personal life is really good for work life balance IMO.