voltage drop & resistivity calculation
26 AWG is 40 ohms / 1000 foot as per this table: -



A 50 foot length of this wire (2 ohms) with 8A will dissipate 128 watts i.e. it doesn't sound a sensible choice. The volt drop at 8A will be 16 volts.

As for your formula I can't tell whether it agrees or disagrees with whatever reference you have.