If the resistance is 0.1 ohms and the current is 0.1A the voltage drop would be .01V correct?
Opps, right, I thought it's 1 OHm. Probably, you should make it 1 ? Moving resistor to gnd path, you may select internal voltage reference of the ADC , and get 5x ( or 2x) better accuracy. I'm not ready to comment on ATTINY ADC internals, if there is a PGA?