4-20 mA sensor with logarithmic scaling
I suspect that there is an error in the table you have posted. The reading goes up by 4 mA per decade except the first one. That should be 4 mA = 0.2 µS/cm. Assuming that is the case then:

mA    µS/cm
 4       0.2
 8       2.0
12      20.0
16     200.0
20    2000.0


Therefore

$$ \mho = 0.2 \cdot10^{\frac {i-4}{4}} $$

where i is the current in mA.



Now that the make and model have been posted (without links to datasheets) it appears that the unit has a five decade range. See InPro 7000, page 6. The only mention of 4 - 20 mA is in section 8.3 of the M300 meter manual but there is no explanation of the scaling.


  The Aout type can be Normal, Bi-Linear, Auto-Range or Logarithmic. The range can be 4–20mA
  or 0–20mA. 


For 5 decades the scaling would have to be 3.2 mA/decade.

mA        µS/cm
 4.0       0.02
 7.2       0.2
10.4       2.0
13.6      20.0
16.8     200.0
20.0    2000.0


For this setup

$$ \mho = 0.2 \cdot10^{\frac {i-4}{3.2}} $$

where i is the current in mA.