在构成列表框项目的两个控件之间均匀分割宽度？

I'm trying to setup a ListBox so that every item has a textblock and a combobox, split evenly across the width of the listbox but I can't seem to find the magic combination of ColumnDefinition properties to do it. Here's my DataTemplate for the listbox item. I've cleaned it up since it was wrong, anyhow.I'm trying to setup a ListBox so that every ite