Ohm's law still applies. Precision may not be as high with one method vs another. Commonly used in the field calculations assume a certain temperature (30C I believe) to come up with a factor that is kind of universally used to come up with the resistance portion in Ohm's law which is considered good enough in most calculations. If a situation where more precision is desired or required then more goes into getting more precise on the actual resistance in said application.
