Assuming you have established whether you are regular or goofy:
Even if your board is "set back" I would suggest you center your bindings on your board.
After you have established your stance width ( generally 20 to 22 inches from center to center of your bindings ) measure from the tip to the middle of your front binding and then do the same from the tail to the center of your back binding. Make sure the measurement is the same.
If you are riding a true twin, this ensures you are riding in the middle of the board. If you are on a board with a built in "set back" you can play with that after you get comfortable on your new board.
Stance Angles are a really, really personal thing, but generally a good place to start is a "Duck Stance" of about + 12 & - 12.
Get it. Wax it. Ride it.