Mr. stevecham
The speaker's 8 ohms is not a part of this calculation.
Your first par. adds the speaker impedance with the cable resistance (like
adding bananas with apples!). The concept is wrong.
The speaker's cables resistance is not part of the load (speakers). They are an
extension of the amplifier, just as I claimed before.
Your second par. is also wrong! DF is always related to 8 ohms (@1kHz), even if
the actual speaker to connected is other (4 ohms or higher than 8 ohms). It is a fix number = 8.
In overall, your way doesn't fit the actual relations, nor explain what
different cables sound different. As if you would be right, all cables would
sound the same.