This is a question with no exact answer, since the lateral stress ratio is rarely known.
Chapter 10: In situ and induced stresses discusses this topic in detail. Hoek's Corner provides links to the individual chapters of Practical Rock Engineering.
Other papers on determining in situ stress, include the following:
Sheory, P.R. (1994). A Theory for In Situ Stresses in Isotropic and Transversely Isotropic Rock. Int. J. Rock Mech. Min. Sci. & Geomech. 31(1)23-34
Stacey, T.R. and Wesseloo, J. (1998). In situ stresses in mining areas in South Africa. Journal of the South African Institute of Mining and Metallurgy. November/December: 365-368
Stacey, T.R., Xianbin, Y., Armstrong, R., and Keyter, G.J. (2003). New slope stability considerations for deep open pit mines. Journal of the South African Institute of Mining and Metallurgy. July/August: 373-390
Grov, E. (2006). The importance of in-situ rock stress in design and construction of sub-surface opening. International Symposium on Utilization of underground space in urban areas. 6-7 November, Sharm El-Sheikh, Egypt.
The above mostly deal with rocks. For soils, Ko=(1-sin(phi)) could be a value to start with if the soil is loose and normally consolidated. Another option is to start with Ko=1 and perform a sensitivity analysis by varying it to see the effect. Starting with Ko as a function of the Poisson ratio may not be the best method, as we have found that this causes weak materials to be initially close to failure, causing a number of modeling issues.