Of course, the best idea is to iterate until there is only one very big block. Since the number of atoms in any real sample of material is very large, this is more or less equivalent to finding the long range behaviour of the RG transformation which took (T, J) → (T ′, J ′) and (T ′, J ′) → (T", J"). Often, when iterated many times, this RG transformation leads to a certain number of fixed points.