Each ski has a reccomended mounting point. This will be marked on your skis in a variety of ways, a notch or a dimple in the sidewall or a mark on your topsheet. These marks aren't always spot on, so it's worth measuring your skis, and dividing that number by 2, to give you center. Then come on here ask what the reccomended mount is on you skis in relation, and work back from there. For example on my new skis I know reccomended is -3 from center, therefore I mark 3 centimeters back from center on my topsheet with some tape. Usually you will find that your reccomended mount and that on the topsheet agree.
Once you've decided where reccomended is you can either decide to go +/- of reccomended. This is very much personal preference, but for your first set of powder skis, I'd reccomend going with rec. mount. If they're a pair of jibby pow planks, this will feel adequately far forward, and you shouldn't have too much of a problem adjusting from your center mounted park skis.