High Tc Josephson nanoJunctions (HTc JnJ) made by ion irradiation have remarkable properties for technological applications. However, the spread in their electrical characteristics increases with the ion dose. We present a simple model to explain the JnJ inhomogeneities, which accounts quantitatively for experimental data. The spread in the slits width of the irradiation mask is the limiting factor.Monte Carlo simulations have been performed using different irradiation conditions to study their influence on the spread of the JnJ charcateristics. A universal behavior has been evidenced, which allows to propose new strategies to optimize JnJ reproducibility.