Here is a simple calculator I use for doing VD on site lighting. The red is user entry fields, blue is calculated. VA is your load, the rest is self explanatory. Just put in single run length, the formula already calcs the return run. I forget what percentage I shot for on my truck but I think it was around 1%. There is an online VD calculator too somewhere. If the MFG is saying a certain footage of a certain gauge wire, just reverse engineer what their VD specs are using a 8.3A(1000W @ 120V) load and go from there.