I'm not sure what a PGA is, but attiny has 1.1V or 2.56V internal reference, so if I used the 1.1V that would definitely help improve accuracy. And unless I am mistake making resistor value would also help with accuracy so ya I see no reason not to make it 1 ohm or even 10 ohm. Of course the higher the value the more power it has to dissipate but as long as the resistor is rated correctly not an issue.
So if I made it 1.1V and used 1 ohm resistor I would expect to see 30 ticks at 0.1amp or a 10 oihm would give 310 ticks
Could I set internal voltage ref and yet leave the resistor on the side it is currently? Since I am only looking at the difference between the two voltages?