There will be too much variation to do this calculation accurately without more information. As air temperature, soil type, soil permeability, presence and abundance of vegetation, vegetation type etc. will influence the relationship. Although previous publications have investigated this relationship, see below for a few examples:
Article Investigating Relationship Between Soil Moisture and Precipi...
Article Soil Moisture Variations in Response to Precipitation Across...
Article Profile distribution of soil moisture response to precipitat...