Native vegetation in private lands plays an important role in providing ecosystem services and safeguarding biodiversity worldwide. Legal protection rules of this vegetation are thus crucial. In Brazil, since 1934, there has been a requirement for landowners to preserve a percentage of their land covered with native vegetation, the Legal Reserve. However, the 1934 legislation applies only to arboreal vegetation, while the protection of savannahs and grasslands begins after 1965 and 1989. The reference year for the application of Legal Reserves can thus have significant impacts on the amount of native vegetation legally protected. Here we tested the effects on Legal Reserve cover in São Paulo State, considering two different legal benchmarks as starting points: 1934 and 1965. Given that the 1934 law precedes the first aerial mapping in Brazil, we developed a methodology to estimate past native vegetation cover through a probabilistic approach. The forest deficit considering 1934 as the initial point is 3% lower than the one starting with the 1965 benchmark. Comparing both scenarios, the 1934 led to a reduction of 1255 farms with Legal Reserve deficit from a total of 30,417. For both scenarios, Legal Reserve deficits were concentrated in the West, Northwest, and Mid-west regions of the state. The outcomes showed that the benchmark change does not significantly affect the total area of Legal Reserves protection, the number of farms potentially benefited by this mechanism, and the amount of native vegetation deficit. However, the use of the 1934 as an initial date for considering protection of Legal Reserve can delay the implementation of the law due to a time-consuming farm-by-farm analysis, once the probabilistic map has an intrinsic limitation for an automatic process. Thus, the environmental gains with the adoption of 1934 as the initial date do not overcome the limitations of using a probabilistic map, suggesting that effective law enforcement depends on reliable and more recent baselines, allowing semi-automated analyses. This scientific evidence can be adopted by the other Brazilian States that have not yet defined the initial legal benchmark, balancing the advantages and disadvantages of adopting the 1934 Forest Act.