Here's a quick and very easy way to find out what your stance is:
Standing flat-footed, jump up and down two or three times (slowly and concentrating on staying under control), pausing a moment between each jump. If you land with good balance and under control each time, after two or three landings you'll find yourself in just about your perfect stance.
Now do it right next to your board, use your feet position as a reference and put your bindings on your board in the same width and angles as your feet. Finally, keeping your bindings at the same width and angles, attach them to your board either slightly set back (back binding is slightly closer to the back tip than the front binding is to the front tip) for all-mountain or more centered for park.