jquery: scroll to an element within an overflowed div

Question

i have 20 list items inside of a div that can only show 5 at a time. what is a good way to scroll to item #10, and then item #20.

i know the height of all the items.

the scrollTo plugin does this, but its source is not super easy to understand without really getting into it. important - i dont want to use this plugin

lets say i have a function that takes 2 elements $parentDiv, $innerListItem

neither $innerListItem.offset().top nor $innerListItem.positon().top gives me the correct scrollTop for $parentDiv