On my projects, I click on the item, or the text box (if item is a text), then click on ARRANGE, ALIGN, and ALIGN MIDDLE. That set the item in the vertical middle of the slide. From there, you make sure the items on top and bottom are in the right place to make it look right. I guess what I am actually doing is aligning the top and bottom equally from the middle, but it works, and self adjusts depending on projection size.