Fin placement really depends on our average back foot placement when you pop up. You want the fins slightly behind your rear foot for a twin set.
Somewhere around 7" - 7.5" up from the tail is a good place to put the trailing edge of the twin fins. For keels you want them back another inch since they have longer bases so 6" - 6'5" up from the tail.
Twin fins are supposed to be loose so you'll want 5 degree cant and 3/16" toe-in. Keels are more of a down the line fin and need less toe-in and cant. 1/8" Toe-in and 2-3 degree cant is good for them.
Be advised using standard side fins from a thruster set for a twin will not work as well as real twin fins. There is more surface area on twin fins that hold you in the wave, a standard thruster side fin will be difficult to make waves on.