1. A method of determining oil based mud contamination of a hydrocarbon fluid obtained from a wellbore during formation testing, the method comprising: receiving a reservoir model for simulating the contamination using a microprocessor; receiving an empirical equation for computing the contamination using the microprocessor wherein the empirical equation for computing the contamination comprises Fitted_Optical ⁢ _Density = C - D × t - 5 12 wherein t equals time, C and D are solved by using a parameter estimation method to fit acquired data and the percent of contamination is computed by: Contamination = 100 × C - Fitted_Optical ⁢ _Density C ; for a first pumping time, separately generating a simulated contamination based on the reservoir model and an empirically computed contamination based on the empirical equation contamination using the microprocessor; comparing the simulated contamination and the empirically computed contamination using the microprocessor; selecting one of the reservoir model and the empirical equation as more accurate and another one as less accurate for the first pumping time using the computer; and revising the less accurate one of the reservoir model and the empirical equation based on a result of the comparing.