thank you for suggestions. I found the answer myself The point is to really use FORMAT() funciton, but the percentage needs to be divided by 100, before formating.
format(@fiteenPercentage / 100, 'P2')
If divided, the outcome is
26-50: (343474) 3,37 %
If not the outcome is
26-50: (343474) 337,00 %
which means the function needs to work with number between 0 and 1