The major subject of our study is the accuracy of contact angle calculations. Reporting new simulation data for graphene-water systems, we show that the majority of previously reported data should be treated with caution, since the proper contact angle can be recorded only after a sufficiently long simulation time. It has been proven that-if one wants to gain accuracy greater than 0.1°-long calculations (exceeding 50 ns) are required. Finally, we also show, using both a Groningen Machine for Chemical Simulations (GROMACS) package and our new molecular dynamics (MD) code, that the changes in the contact angle, caused by graphene bottom layer rotation, are within the range of calculation error. We also propose a novel definition of the bottom of the droplet as the height where the density is half the density of liquid water. This new definition is applied in the method of the contact angle calculation from the MD simulation data.