How can I programatically determine how many items will be in a list box based upon the size of the list box and the current font?

I'm trying to create a "filing cabinet" for records in my app, and I want to let users drag-and-drop items from one folder to another.

I have the list of folders on the left, and the records on the right. I want a user to be able to select a folder, then highlight one or more records, and drag them into a new folder. To do this, I need to know which item in the folders list box the drag-and-drop ended in.

I know that I have the x and y coordinates of this value, but how can I figure out which row this point falls in?

Thanks for any suggestions anyone can give

--Sam