Modify a current range

[Luis]

Hello!
I'm working with a cell that mesures currents between -25uA and 25uA and I'm having some thouble with signal resolition. The range 100uA is ok but, for me, it will be much better if I've a got a range of +/-30uA, for example.
Could you send me the resistor combination for modify the +/-1000uA range (the less important for me) to +/-30uA?
If you could indicate how this calculation can be done, will be very helpfully.

Many thanks for your help!