They work it out in full monthly payments if I remember correctly from a car I swapped early.
£3249 divide 36 = £90.25 (month)
you’ve had the car 13 months this month so
13 x £90.25 = £1173.25
£3249 – £1173.25 = 2075.75
edit: I think they changed it so you might get a pro rata good condition bonus too, but I’m really not sure about that because it wasn’t when I changed a couple of cars ago.