Model silicone networks obtained by hydrosilation of vinyl terminated polydimethylsiloxane chains have been used extensively to verify molecular theories of rubber elasticity. In these networks, the maximum extent of reaction obtained during cross linking considerably affects their final structure. The degree of completion of the hydrosilation reaction depends on the concentration of reactive groups. A recursive approach and kinetic data from the literature are used to show that, for this particular reaction, perfect networks, i.e. those with high concentration of elastically active network chains, are only obtained when a relatively high concentration of reactive groups is present in the system. This critical concentration corresponds in a bulk reaction to polydimethylsiloxane chains with a number average molecular weight lower than 10,000.