We report on our Monte Carlo calculations of the conductivity of monosized and conducting spherical particles dispersed in a homogeneous matrix, with interparticle transport mechanism given by electron tunneling. We show that our numerical results can be reproduced by a simple formula based on a critical path analysis and ξ which gives also a practical way to estimate the characteristic tunneling length in real composites. We find that ξ is about 1 nm for several low structure carbon black polymer composites, in agreement with the expected order of magnitude.