  It has been shown recently that the background independent open string field
theory provides an exact description of the tachyon condensation on unstable
D-branes of bosonic string theory. In this analysis the overall normalisation
of the action was chosen so that it reproduces the conjectured relations
involving tachyon condensation. In this paper we fix this normalisation by
comparing the on-shell three tachyon amplitude computed from the background
independent open string field theory with the same amplitude computed from the
cubic open string field theory, which in turn agrees with the result of the
first quantised theory. We find that this normalisation factor is in precise
agreement with the one required for verifying the conjectured properties of the
tachyon potential.
