How to set UIWrapContent defalut position ? (NGUI)

Hi sorry to bother you.

currently i want use UIWrapContent to make an list (300+ item, and all item have same width&height).

follow the example, i have successful make the list display≻roll. but each time the list is start from 0 (realIndex).

how do i set the default display index?

e.g. make item realIndex = 120 show in the middle of the scrollview.

if the scrollview can display 3 item . so ,when i click play button in Unity Editor. should display 121,120,119.

for now it display 1,0,-1.

ps: my setting for the UIWrapContent is

Item Height : 200 Range Limit 0 to 300 Cull Content = true;

ScrollView:
Offset x=0 y=0 Center x=0 y=0 Size x=200 y=600 Softness x=4 y=4

Thanks