So you mean modulation and not the parameters value it self.
Yes it's true, you can't directly set a 3 decimal value. But you can use the sidechain. In your image you use Random 1 as a modulator. If you in example use the same Random 1 as sidechain for that same modulation, then try to set the modulation value to 0.01 and set the sidechain value to -0,50.