Does the NEC have a table for oversizing wire for extended lengths?
Southwire has a voltage drop calculator here:  https://www.southwire.com/calculator-vdrop
The resistance for 250' of 12ga is indeed around .4 ohms. 

You also should remember that you'll only be drawing 13 amps when the saw is fully loaded. According to the calculator when drawing the full 13 amps you'd see a voltage drop of almost 10% on 12 ga wire. For 10ga at full load it would be just over 5%.  

Up to you if you want to save some money on wire and risk higher voltage drop or spend quite a few more bucks and get 10ga.