JT:
If your speaker cable is 50 feet long, the signal travels 100 feet to make the round trip. If the gauge of the wire is 10 AWG and has a resistance of 1 ohm per 1000 feet, the 100 foot round trip gives a resistance of 0.1 ohm.
In the table I referred to, the round trip resistance is given, so those particular resistance values do not need to be doubled when considering the amp-to-speaker cable run. If you were to read resistance values from a table that did not already take into account the round-trip, then a 50 foot cable run would involve calculating for 100 feet of the conductor.