honestly, go a littler longer than you would think. at least head high, especially if they are light skis.
Why you ask?
-for one, a twintip ski rides about 20 cm shorter than a normal ski of the same length, so a longer ski gives you more effective edge, which means more stability, better edge hold (even if your edges are detuned)
a longer ski (except 4frnts) have a wider sidecut radius, and are more stable. which, again, is good for landings
a twin tip ski is pretty center mounted, and they feel shorter than they are.
a longer ski is faster (more base area), and are way better for landings.
if all you are doing is rails, go shorter. but for your general all mountain riding, head high is a good place to go.
opps, that was free advertising.