I measured my Yellowstone Solo. Total length from the outside ends of the deck plates is 168" (14'). Center is 84". From the outside edge of the front deck plate to the front edge of the seat is 87", so it is 3" back from the center.
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Yes, someday I need to fix the caning on that seat.
Run the tape along the gunnel from the front of the deck plate to the screw holding the seat hanger is just shy of 89".
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Probably a little shy because the metal tape doesn't follow the gunnel perfectly.
As a general rule, you want the front plane of your body to be just behind the center of the boat so you will have neutral to slightly stern heavy trim. I have seen recommendations for whitewater pedestals to be 10" to 12" back from the center, but I think that is more the knelling position than the front of the pedestal. In my Outrage, the front of the pedestal is actually in front of the center line, but when I kneel in it, the front of my body is pretty much just behind the center.
With a bench seat, you need to be in trim when you are kneeling (slightly in front of the front edge), and sitting (slightly behind the front edge). 10" back might be OK when you are knelling (like with a kneeling thwart), but I think you would be out of trim sitting. 3" is probably a happy medium. If you are putting a sliding seat in I guess it doesn't matter - you can put it where it needs to be.
Good luck adding the new seat.