Lewis,
That's pretty much old school and works but I've also seen reference to three (3) additional and as high as four (4) extra feet. You want enough slack so when you turn hard to port or starboard you don't kink the cables.(we are talking outboard here?)
I ran a wire snake (fish tape) from the engine( cranked hard over to starboard) to the binacle control and made sure the bends were as gentle as possible. Marked the snake with tape, pulled it out, measured and purchased the next higher length. Better to have a little too much than not enough. You can always hide a little under the console or let it gather in the transom.
Good luck.
