One thing you could do is calculate placement the old fashioned way: find the midpoint of the running surface on the ski, and find where the ball of the foot will be in your boot, and match these up. For how to do this (and why this might be more accurate than just going by the lines manufacture's use) click herehttp://www.techsupportforskiers.com/..._placement.htm
--even if you don't buy the whole argument about why manufacuter's marks might sometimes be misplaced, this solves the problem of how to mount a park ski for regular skiing.
(Just to clarify: the article linked to is about where manufactuer's marks on NON-park skis put you, so it talks about a mounting position slightly foward of where the manufacuter's marks put you. Since you're talking about a park ski you'll end up in back of where the mark would put you, but the discussion of how to calculate where you should be still applies--note that because the calculations are based on running surface they already take into acct the fact that your skis are twin tips.)