The turbidity of the liquid-liquid mixture perfluoroheptane and 2,2,4-trimethylpentane (also known as iso-octane) has been measured and used to determine the correlation length amplitude xi(0). By measuring the ratio of the transmitted to incident light intensities over three decades in reduced temperature, we are able to determine that xi(0)=0.253+/-0.005 nm. This value differs significantly from that reported in the literature. The earlier literature value for xi(0), when combined with the amplitude A(+) determined in a recent heat capacity measurement on this system, produced the two-scale-factor universality constant X=A(+)xi(0)(3)/k(B)=0.009+/-0.002, which was not consistent with other experiments and theories that had found a value between 0.018 and 0.020. However, when we combine the value of xi(0) determined in this experiment with the amplitude A(+) then the two-scale-factor universality constant X=0.020+/-0.002, which is quite consistent with other determinations.