The calculation is sometimes not that simple:
If you buy a module with a level shifter, then there is often a 10k before the level shifter and 10k after the level shifter.
The current the pull the SDA or SCL down has to pull down both sides of the level shifter.
5V / 10k = 0.5mA
3.3V / 10k = 0.33mA
That results already in a current of 0.83mA.
By the way, you don't have to select a "safe" value of 1 or 2mA. If the resulting current is 3mA, then it is okay.
It is possible to make a sketch with a idle I2C bus. Just a Wire.begin() in the setup().
Then you can measure the shortcut current from SDA to GND and from SCL to GND with a multimeter. That should be close to what you have calculated.