Hi,
it is as unprecise as cheap.
Because the thermal dirft of copper is relatively high and the precision depends on etching and galvanic tolerance.
Also you have to take care about stray inductance. With about 250mm trace length this is not negligible. Not good for peak currents and high frequency signals.
As i can see it is about 100mOhm with 1oz / 35um copper thickness as you wanted.
* The price of PCB area may be more than you safe by a extra resistor. But the resistor is way more precise.
* to calibrate your resistor may cost more than a resistor.
You could even use a 20mohms resistor with 0.2W and amplify the signal by 5 and get more reliable results.
Usually my goal is precision, therfore it is no option for me, but it can be a solution for you. It depends on your needs/specifications.
Klaus