My suggestion - rather than base it on Van's kit (which assumes a basic VFR steam gauge plane), put together a planning document with a list of all *your* electrical devices, each device's electrical load, and the appropriate wire gauge.
Then make a drawing that shows where each device is located. You can then ballpark the length for each device. From that you can total up your wire needs (add about 40% cuz it always just works out that way). Don't forget ground wires and the large-gauge wires too.
This is part of the planning process for a Vertical Power electrical system and should be too if you're wiring the old-fashion way. You can use something as simple as Excel and Power Point to do your planning.
