An extended ASYNNNI model, that beside nearest-neighbour and next-nearest neighbour O-O interactions in the basal plane also includes interactions between the three nearest oxygen atoms, is used to describe the statistics of CuO chain fragmentation and to calculate doping and T
c in YBa2Cu3O6+x
. Calculations were made by the Monte Carlo method employing the recently proposed charge transfer model that assumes only chains whose length is equal to, or exceeds, a characteristic (critical) length, l
cr, can provide holes to the layers and contribute to doping p. The obtained p(x) is then combined with a universal T
c versus p relation to yield T
c(x) characteristics that correlate remarkably with those reported in recent experiments. The best coordination between theoretical and experimental T
c(x) characteristics has been achieved for l
cr = 2, implying that only isolated basal plane oxygen atoms (trivial chains) do not contribute holes to CuO2 layers.