How calculate the input offset current effect on measured voltage?
Input offset current is an input referred term that develops a voltage across the input resistors, and is usually calculated for a no-input condition (so a static analysis).

The input offset current is precisely what it says: the difference in input current between the two inputs. In this case, that difference in input currents will develop a voltage across the input resistance.

In your case, assuming Rdiff is very large compared with the sense resistor, you have a worst case offset current of 200pA * R1.

That yields a +/- 50nV voltage difference at the inputs of the amplifier. This will be multiplied by the gain of the amplifier to yield an output offset error.

See this article for a thorough discussion (search for offset).

Update: made notes about the effect in this circuit.

Your circuit is measuring a 4 - 20mA loop. Even at minimum current of 4mA through the sense resistor, the error due to input offset current is -146dB (or 0.05 parts per million, if you prefer). With such a small error relative to the current you intend to sense, it is a non-issue here.

You are doing the right thing, though, as input offset voltage, input offset current and input bias current are all sources of errors and they can be a cause of difficulty in high gain circuits.