Use string to get a rough idea of what you need, then give yourself an extra couple of feet to play with. All of the little twists and turn around body contours can eat up a lot of length of cable.

Also be absolutely sure of where you want to put your equipment before you get your cable. I'm personally not a big fan of under the seat mounting for a few reasons and I have only done it with small amps. I don't know how big the 1200.1 is but most amps with that kind of power take up more than a bit of space physically in addition to whats needed for proper ventilation. Just my $.02